Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it.
- Optics studies the behaviour of electromagnetic radiation in the visible, ultraviolet, and infrared regions of the electromagnetic spectrum.
- Some fields of optics also study the behaviour and properties of other forms of electromagnetic radiation such as X-rays and microwaves.
- The observation and study of optical phenomena offer many clues as to the nature of light.
- Optical phenomena include absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
- Optics explains the appearance of rainbows, how light reflects off mirrors, how light refracts through glass or water, and why light separates into a spectrum of colours as it passes through a prism.
Optical illusions and other visual anomalies are caused by the way the human visual system processes information.
- Physical illusions: Physical illusions result from the limitations and assumptions of the human visual system when interpreting the external world.
- Physiological illusions: Physiological illusions are often connected with the different attributes of visual perception and occur when visual stimuli are beyond our brain’s processing ability.
- Cognitive illusions: Cognitive illusions result from the brain’s inability to correctly interpret visual information, leading to uncertainties or errors in perception.
The optic radiation consists of neural tracts formed by the axons of neurons in the lateral geniculate nucleus (LGN), which project to the primary visual cortex.
- There are two optic radiations, one on each side of the brain, each responsible for carrying visual information to the corresponding hemisphere.
- The optic radiation is divided into two main pathways:
- The upper division, which carries information from the lower visual field,
- The lower division (also called Meyer’s loop), which carries information from the upper visual field.
- These pathways ensure that visual information is accurately mapped to the primary visual cortex, where it is processed and interpreted.
The optic nerve in the human eye is a cable-like bundle of nerve fibres composed of the axons of ganglion cells, responsible for transmitting visual information to the brain’s lateral geniculate nucleus.
- This nerve contains about a million fibres that carry a constant stream of visual data, received from the eye’s photoreceptors—rods and cones—as well as intermediate neurons such as bipolar and amacrine cells.
- The optic nerve functions like a parallel communication cable, with each fibre transmitting distinct information about light intensity and patterns from specific regions of the visual field, allowing the brain to construct a cohesive image of the surroundings.
- The optic nerve exits the eye at a spot called the optic disc, where no photoreceptors are present, creating a natural “blind spot” in the visual field. The brain compensates for this by filling in the missing information.
- Some fibres from the optic nerve cross over to the opposite side of the brain at the optic chiasm. This crossover allows visual information from both eyes to be processed in both hemispheres of the brain, which is crucial for depth perception and a unified field of vision.
The optic chiasm is the part of the human brain where the optic nerves partially cross. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus.
- The cross-over of optic nerve fibres at the optic chiasm allows the visual cortex to receive the same hemispheric visual field from both eyes.
- Superimposing and processing these monocular visual signals allow the visual cortex to generate binocular and stereoscopic vision.
- For example, the right visual cortex receives the temporal visual field of the left eye, and the nasal visual field of the right eye, which results in the right visual cortex producing a binocular image of the left hemispheric visual field. The net result of optic nerves crossing over at the optic chiasm is for the right cerebral hemisphere to sense and process left hemispheric vision, and for the left cerebral hemisphere to sense and process right hemispheric vision.
Opacity refers to the extent to which an object or surface prevents light from passing through, thereby obstructing light from reaching objects or space beyond.
- Opacity is influenced by factors such as absorption, reflection, and scattering of light. An entirely opaque substance reflects and absorbs all incident light, allowing no transmission or scattering.
- When light strikes the boundary between two media, some of it is reflected, some is absorbed, and some is scattered. The remaining light is refracted and transmitted through the second medium. Opacity measures how effectively this second medium obstructs the passage of light.
- An opaque object is neither transparent (which allows all light to pass through) nor translucent (which permits partial light transmission).
- The opacity of certain materials can vary with the wavelength of light. For instance, some types of glass are transparent to visible light but opaque to ultraviolet radiation.
A nuclear reaction involves changes within the nucleus of an atom, resulting in the release of energy and often the emission of particles, as well as electromagnetic radiation. This radiation can span various parts of the electromagnetic spectrum, with gamma rays being a particularly common form.
- Here’s a breakdown of how nuclear reactions can be sources of electromagnetic radiation:
- Nuclear Fission: When the nucleus of a heavy atom splits into smaller nuclei, it releases a significant amount of energy. A significant portion of this energy is emitted as gamma rays, which are high-energy photons within the electromagnetic spectrum. Nuclear power plants and atomic bombs harness fission reactions.
- Nuclear Fusion: When the nuclei of lighter atoms combine to form a heavier nucleus, it also releases energy. In stars like our Sun, nuclear fusion releases large amounts of energy, including a range of electromagnetic radiation from infrared light to ultraviolet light, and even gamma rays.
- Radioactive Decay: Unstable atomic nuclei undergo decay and change their composition to reach a more stable state. During this process, they can release charged particles (like alpha or beta particles), neutrinos, and often gamma rays.
If one line is normal to another, then it is at right angles to it.
In geometry, normal (a or the normal) refers to a line drawn perpendicular to a given line, plane or surface.
- How the normal appears in a geometric drawing depends on the circumstances:
- When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to the surface, forming a right angle (90°) with it.
- Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
- When dealing with curved surfaces, such as those found on spheres or other three-dimensional objects, determining the normal requires a slightly different approach. Instead of simply drawing a line perpendicular to the surface as with a flat plane, draw the normal straight up from the point where light hits the surface.
- When considering a sphere, the normal line passes through the centre of the sphere. This is because, regardless of where light enters or exits the sphere, the normal represents the direction perpendicular to the surface at that point.
A non-spectral colour is a colour that is not present in the visible spectrum and cannot be produced by a single wavelength or narrow band of wavelengths of light.
- While spectral colours are evoked by a single wavelength of light in the visible spectrum, non-spectral colours are produced by a combination of spectral colours from different parts of the spectrum.
- Colours evoked by a single wavelength of light are often described as being produced by monochromatic light.
- Magenta, pink, cyan and brown are examples of non-spectral colours produced by combining different wavelengths of light:
- Blue and red = magenta
- Red and purple = pink
- Blue and green = cyan
- Red, yellow and blue = brown
Newtonian mechanics is a branch of physics that describes the motion of objects under the influence of forces. It is based on the three laws of motion developed by Isaac Newton in the 17th century.
The three laws of motion are:
- An object at rest will remain at rest, or if in motion, will remain at a constant speed and in a straight line unless acted upon by an external force.
- The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass.
- For every action, there is an equal and opposite reaction.
- These laws can be used to describe a wide range of phenomena, from the motion of planets to the behaviour of fluids and the propagation of waves. They are also applied in many fields, including engineering, medicine, and astronomy.
- Newtonian mechanics predicts the motion of objects with high accuracy. However, it has limitations; for example, it cannot explain the behaviour of light at atomic and subatomic levels, where light behaves as both a wave and a particle—something Newtonian mechanics cannot describe.
- Despite its limitations, Newtonian mechanics remains a crucial and useful theory. It is applied in many fields and has greatly deepened our understanding of the universe.
Here are examples of Newtonian mechanics in action:
- When you throw a ball, the ball accelerates due to the force of gravity.
- When you ride a bike, you need to pedal to keep moving forward because of the force of friction.
- When you sit in a chair, the chair exerts an upward force on you that balances the downward force of gravity.
- When you jump off a cliff, you accelerate due to gravity until you hit the water.
Neurons are specialized cells that transmit electrical and chemical signals throughout the brain and central nervous system, enabling communication between different parts of the body the central nervous system.
- Neurons are the electrically excitable cells that are the fundamental building blocks of the central nervous system of human beings.
- Neurons interconnect the systems and organs that maintain the body’s essential functions.
- Neurons send and receive signals that allow us to sense the external world, move, think, form memories and much more.
- Neurons are of three principal types: motor neurons, sensory neurons and interneurons.
- Neurons connect together via specialized filaments called synapses.
- In the neocortex (making up about 80% of the human brain), approximately 70-80% of nervous tissue is in the form of neurons whilst the remainder is composed of interneurons.
Nature, in the broad sense, refers to the physical universe encompassing all living organisms (plants, animals, microorganisms) and non-living entities (such as rocks, water, and atmospheric elements). It includes the natural processes and forces that govern the physical world, as well as ecosystems and the interactions between living and non-living components.
- Nature, in the broadest sense, refers to the physical universe, encompassing all living and non-living things.
- In a more limited sense, nature can refer specifically to interconnected living organisms, including plants, insects, and animals, while sometimes excluding non-living elements like oceans, continents, and climate.
- However, it’s important to note that non-living phenomena are also an essential part of nature, as they play a vital role in ecosystems and the natural processes that sustain life.
- The concept of nature is complex and multifaceted. For instance, while humans are part of nature, human-made environments such as cities, agriculture, and industries are often viewed as distinct from other natural phenomena.
A natural light source refers to any source of light that occurs in nature and is not created by human activity.
- The Sun is the most prominent and important natural light source on Earth, providing sunlight that powers life, such as through photosynthesis in plants.
- Stars emit light naturally due to nuclear reactions in their cores, which generate massive amounts of energy released as light.
- Fire can occur naturally through processes like lightning strikes igniting dry vegetation or volcanic activity.
- Bioluminescence is the natural emission of light by living organisms such as fireflies, some fungi, and deep-sea creatures.
- Auroras (like the Northern and Southern Lights) are natural light displays in the Earth’s atmosphere, caused by the interaction of solar wind with the Earth’s magnetic field.
- Lightning is another natural light source, produced during electrical storms when electrical charges in clouds discharge.
- Natural light sources vary in brightness, spectrum, and duration.
Müller glia, or Müller cells, are a type of retinal glial cells in the human eye that serve as support cells for the neurons, as other glial cells do.
- An important role of Müller cells is to funnel light to the rod and cone photoreceptors from the outer surface of the retina to where the photoreceptors are located.
- Other functions include maintaining the structural and functional stability of retinal cells. They regulate the extracellular environment, remove debris, provide electrical insulation of receptors and other neurons, and mechanical support of the neural retina.
- All glial cells (or simply glia), are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system.
- Müller cells are the most common type of glial cell found in the retina. While their cell bodies are located in the inner nuclear layer of the retina, they span the entire retina.
A nanometre (nm) is a unit of length in the metric system, equal to one billionth of a metre (1 nm = 1 × 10⁻⁹ metres). It is commonly used to measure extremely small distances, particularly at the atomic and molecular scale.
- In the context of light and electromagnetic radiation, a nanometre is often used to describe wavelengths of visible light.
The wavelength of visible light ranges from about 700 nm (red) to 400 nm (violet).
- Nanometres are also used to measure components like the thickness of materials, the size of particles in nanotechnology, and the spacing between atoms in a crystal lattice.
Momentum is a measure of how much mass an object has and how fast it is moving. It is calculated by multiplying the mass of the object by its velocity.
- Momentum is a vector quantity, which means that it has both magnitude and direction.
- Momentum = mass x velocity.
- Momentum is conserved, which means that the total momentum of a system remains constant unless an external force acts on the system.
- Momentum can only be transferred between objects, not created or destroyed.
- Examples of momentum:
- A bowling ball has more momentum than a baseball because it has more mass.
- A car moving at 60 mph has more momentum than a car moving at 30 mph.
- A rocket launching into space has a lot of momentum because it has a lot of mass and it is moving very fast.
Monochromatic refers to light or electromagnetic radiation that consists of a single wavelength or frequency. In simpler terms, monochromatic light is composed of just one colour. The term comes from the Greek words mono (meaning “one”) and chroma (meaning “colour”).
- Monochromatic colours are created by using variations of a single hue, incorporating both shades (by adding black) and tints (by adding white). For example, a monochromatic colour scheme could involve a range of blues or pinks.
- It’s important not to confuse monochrome with greyscale. Monochrome refers to variations of a single hue, which can include any colour. In contrast, greyscale consists solely of shades of grey, with no colour information.
- In physics, monochromatic light refers to visible light or other electromagnetic radiation that has a single wavelength or frequency, resulting in light of a single colour.
- A surface or material is considered monochromatic if it features only one hue or a combination of tints and shades of the same colour.
The term metameric refers to visually indistinguishable colour stimuli that appear the same but have different spectral compositions are called metameric.
- Metameric stimuli are colour stimuli that are indistinguishable from one another because they produce the same responses from the three types of cone cells in the human eye that are responsible for colour vision.
- Metameric matches can occur in different parts of the spectrum, which means that the spectral power distributions of different light sources can look similar, but not identical, to one another.
- A class of metameric stimuli can be specified by a set of tristimulus values, which represent the amounts of three reference colours, typically red, green, and blue, in a given trichromatic system, that are required to match the colour of the stimulus considered.
- The most important application of metameric stimuli is in the use of tristimulus values for additive colour mixing, such as in computer displays and TVs.
- The RGB colour model, for example, uses mixtures of red, green, and blue light to produce a wide range of colours visible to an observer.
In physics and optics, a medium refers to any material through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.
- Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
- The properties of the medium, such as its density and composition, influence how light propagates through it.
- Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
- Examples of Mediums:
- Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
- Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
- Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.
Matter is anything that has mass and energy and occupies space by virtue of having volume.
- Matter is the substance that makes up all physical objects and substances in the universe, including solids, liquids, and gases.
- Matter is made up of atoms, which may combine to form molecules. Atoms in turn consist of subatomic particles such as protons, neutrons, and electrons.
- Subatomic particles have mass and may have energy.
- Matter can undergo physical and chemical changes, but the total amount of matter in a closed system remains constant (Law of Conservation of Matter). This means that matter cannot be created or destroyed in chemical reactions, only transformed into different forms.
- Light is a form of electromagnetic radiation, which does not have mass and is not considered matter.
- light interacts with matter (e.g., through absorption, reflection, and transmission), but is not composed of particles with mass.
A material thing is made up of matter, which includes all substances that have mass and occupy space. Matter is composed of atoms and molecules, and its properties include mass, volume, and density.
- Material things include objects, living organisms, and even intangible things such as sound or light, which are considered material because they are made up of particles.
- An attribute of an object is called a property if it can be measured or observed through the senses (e.g. its colour, size, weight, odour, taste, and location).
- Objects can be identified or characterized through their properties, which manifest themselves in various ways.
- These manifestations often exhibit consistent patterns, indicating that there is a underlying cause or mechanism that governs the properties:
- For example when different metals are mixed to form alloys, such as bronze or steel, the resulting material often exhibits a consistent relationship between its composition (the types and proportions of metals) and its density. So increasing the percentage of a denser metal in an alloy tends to increase its overall density.
A material is the substances or matter that a thing is made of.
- Material is a broad term for a chemical substance or mixture of substances that constitute an object.
- Materials are composed of atoms and molecules arranged in various configurations, which determine their properties and behaviour.
- Materials can have natural origins, such as wood, stone, and metals, or synthetic origins, such as polymers and ceramics. Materials can also be classified based on whether they are organic or inorganic.
- The properties of a material depend on its structure at different length scales, from atomic to macroscopic scales.
- Materials can be classified based on physical and chemical properties such as mechanical, thermal, electrical, and magnetic properties.
- Materials are studied in materials science, a branch of engineering that focuses on structure, properties, and processing.
Mass is a fundamental property of matter and is defined as the amount of matter present in an object and is independent of external factors such as location or the presence of gravitational fields.
- A large object made of a given material has greater mass than a small object made of the same material because it contains more matter.
- Mass is not the same as weight because weight varies with gravity while mass remains constant.
- Weight is the force exerted on an object due to gravity:
- An object of a known mass weighs more on earth than on the moon due to differences in gravity.
Luminosity refers to the total amount of light being given off by a source, regardless of the direction.
- The luminosity of a light source depends on the total amount of power it consumes and the efficiency with which it converts that power into visible light.
- Luminosity is a measurable quantity and is often used as an objective measure of the total amount of light being emitted by a source.
- So luminosity refers to the total amount of light emitted by a source per unit time and is often measured in units like watts or lumens. It indicates the raw power of the light source, regardless of its colour or direction.
- The maximum luminosity of a display device corresponds with the brightest white it can reproduce and is called the white point.
- The black point corresponds with the minimum luminosity of a device, so corresponds with the device being turned off.
- The contrast ratio of the maximum and minimum luminosity of a television or computer screen is typically more than 280:1.
Luminance is a measure of the perceived brightness of light reaching the human eye, considering both the amount of light emitted, transmitted, or reflected from a surface and the human eye’s sensitivity to different wavelengths of light. In simpler terms, luminance quantifies how bright a surface appears to the human eye under given conditions.
- Luminance focuses on luminous intensity as experienced by an observer. While luminous intensity refers to the amount of light emitted in a specific direction by a source, luminance measures how bright a surface appears based on both the light it reflects or emits and the observer’s perspective.
- For example, imagine a lamp shining in a dark room. While the lamp emits a certain amount of total light (luminous flux), the actual brightness (luminance) of a wall the light falls on depends on several factors:
- Reflectivity: The wall reflects only a portion of the light that hits it. The amount of light reflected affects the overall luminance the human eye perceives.
- Spectral Sensitivity: Human eyes are more sensitive to green light than to blue or red light. Therefore, even if two surfaces receive the same light, a green surface will appear brighter than a blue one because of the eye’s higher sensitivity to green wavelengths.
- Measuring luminance helps us understand real-world scenarios:
- Moonlight: While not very luminous (doesn’t emit much light), moonlight creates a certain luminance on sand in a desert, allowing us to see our surroundings.
- Road safety: Streetlights need specific luminance levels to ensure safe visibility for drivers, considering both the total light emitted and road reflectivity.
- Book reading: The luminance of a book under a lamp determines how comfortable and clear the text appears to your eyes.
The LMS colour space is a practical implementation of trichromatic colour theory that enables the full range of human observable colours to be specified by measuring the responsiveness of the L, M and S cones to each wavelength of light within the visible spectrum.
- The LMS colour space was one of the first systematic demonstrations of trichromatic colour theory.
- LMS describes how the three types of cone photoreceptors (L, M and S cone types) in a human eye respond given any particular light stimuli.
- The method used in the development of the LMS colour space produced a generalized representation of human colour perception.
- The underlying principle was that any colour can be described in physiological terms by measuring the response of the L, M and S cone cells in the human eye’s retina to different wavelengths of light.
- The initial source of data for the LMS colour space was taken from experiments that compared the spectral sensitivity of subjects with normal sensitivity with other subjects experiencing forms of colour blindness.
- A more recent technique used to collect data for LMS belongs to the field of visual psychophysics and is known as heterochromatic flicker photometry. It provides extensive and accurate spectral sensitivity data obtained from cellular material removed from the eye.
- The LMS colour space describes human observable colours using three parameters, known as tristimulus colour values, each component of which corresponds with the response of the L, M and S cone types.
Light Emission refers to the process by which light (electromagnetic radiation) is produced and emitted by a source. This can occur through various mechanisms, depending on the nature of the source and the conditions involved. These processes involve the transformation of energy into light.
Luminescence
Light is produced when excited electrons within a material drop back to a lower energy state, releasing energy in the form of light. This category includes:
- Photoluminescence (excitation by light absorption)
- Electroluminescence (excitation by an electrical current)
- Chemiluminescence (excitation by a chemical reaction)
- Bioelectroluminescence (excitation through biological processes in living organisms)
- Thermoluminescence (excitation by heat)
- Sonoluminescence (excitation by sound waves and collapsing bubbles)
- Triboluminescence (excitation by mechanical stress)
Thermal Radiation
- Light is produced due to the thermal motion of atoms and molecules. Any object above absolute zero emits thermal radiation, including a portion in the visible light spectrum.
Nuclear Reactions
- Light is produced as a byproduct of nuclear processes like fission (splitting atomic nuclei) and fusion (combining atomic nuclei).
Blackbody Radiation
- A specific type of thermal radiation emitted by a perfect ‘blackbody’. The spectrum and intensity of blackbody radiation are determined solely by the object’s temperature.
Light waves are another name for electromagnetic radiation. They consist of self-propagating waves of electric and magnetic fields that travel through space. This wave motion transports energy but doesn’t involve the movement of physical matter itself.
- The distance between peaks in a wave is known as the wavelength. Different wavelengths correspond to specific portions of the electromagnetic spectrum.
- The visible spectrum, which our eyes can detect, occupies a limited range within this spectrum.
- Within the visible spectrum, red light has a longer wavelength than violet light.
- Light exhibits wave-particle duality. This means that light can demonstrate properties of both waves and particles (photons) depending on the experimental setup.
- Light waves interact with matter through various mechanisms. These include reflection (e.g., bouncing off a mirror), absorption (e.g., conversion of light energy to heat by dark clothing), and refraction (e.g., bending of light as it passes through a prism).
Light stimuli trigger physiological responses in living organisms, such as vision, photosynthesis, and circadian rhythms.
- Different organisms respond differently to light stimuli, depending on the presence or absence of specialized light-sensitive cells or photoreceptors.
- Light that enters the human eye and stimulates the visual system is called a visual stimulus.
- The term colour stimulus is used because the light stimulus produces the perception of colour for an observer.
- Every light stimulus can be described in terms of the composition and intensity of wavelengths of light that enter the eye.
- A light stimulus may consist of a combination of red, orange, yellow, green, blue, and violet wavelengths of light. The colour perceived by an observer is influenced not only by the mixture of wavelengths but also by the intensity of light at each wavelength.
- In many cases, the intensity of light varies across a range of wavelengths, and this variation can be described by the spectral power distribution of the stimulus.
Kinetic energy is the energy an object has because of its motion.
- Planets, cars, people and atoms all have kinetic energy due to their motion.
- When a force is applied to an object, its kinetic energy can change.
- Kinetic energy is the energy of motion, while potential energy is the energy of position or state.
- Most interactions between objects involve forces and can transfer energy.
The lateral geniculate nucleus (LGN) is a relay centre in the visual pathway from the eye to the brain. It receives signals from the retina via the axons of ganglion cells. The thalamus, a part of the brain located near the brainstem, houses the LGN.
- The thalamus which houses the lateral geniculate nucleus is a small structure within the brain, located just above the brain stem between the cerebral cortex and the midbrain and has extensive nerve connections to both.
- The LGN specializes in processing visual information from both eyes. It resolves relationships between different visual inputs, helping us understand the sequence of events and the location of objects in our field of view.
- Some of this processing involves signals from one eye, while others deal with information from both eyes to create a three-dimensional perception of the world. The LGN acts as a central connection for the optic nerve to the primary visual cortex in the occipital lobe. Both the left and right hemispheres of the brain have a lateral geniculate nucleus.
- There are three major cell types in the LGN, each connecting to different types of ganglion cells and playing specific roles in vision:
- P cells: Process information about colour and fine detail.
- M cells: Respond to motion.
- K cells: Involved in low-resolution processing.
A laser is a light source that can create a narrow and intense beam of electromagnetic radiation. Unlike a flashlight, which has a bulb that emits light in all directions, a laser beam focuses its light into a concentrated stream of photons. LASER stands for Light Amplification by Stimulated Emission of Radiation.
- Light waves are made up of tiny packets of energy called photons.
- Normal light emission happens when atoms or molecules release photons when they transition from higher energy states to lower ones. These emitted photons have random directions and energies, creating a diffuse light.
- The concept that makes lasers unique is stimulated emission. This occurs when an incoming photon interacts with an excited atom in the laser material. The photon’s energy triggers the excited atom in the material to emit a new photon with identical characteristics. The new photon has the same wavelength and so colour, phase and direction as the original.
- Laser material refers to the medium that is used to generate the laser light.
- This phenomenon creates a cascade effect. The newly emitted photon can itself stimulate another excited atom, leading to two identical photons travelling in the same direction. This process repeats, rapidly amplifying the initial light within the laser cavity.
- The cavity comprises two mirrors strategically positioned at the opposite ends of the laser material. One mirror is fully reflective, while the other partially reflects.
- As the amplified light bounces between the mirrors, it continues to stimulate more emissions, resulting in an intense beam of identical photons. The partially reflective mirror allows a portion of this intense light to escape as the laser beam, while the rest continues to contribute to the amplification within the cavity.
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current flows through it. Electroluminescence is the process where this happens: voltage applied to the semiconductor makes electrons flow across a junction, releasing energy as light.
- Semiconductors, typically made from gallium nitride, are solid-state materials with unique properties that allow them to emit light at specific wavelengths, determining the perceived colour.
- LEDs typically emit one colour with a narrow range of wavelengths.
- Multicoloured LEDs combine three diodes emitting the RGB primary colours – red, green, and blue light.
- By adjusting the relative brightness of the primary colours, a vast array of colours can be created.
- Combining the three primary colours in equal proportions produces white light.
Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space. Even if humans had never evolved, stars would have emitted electromagnetic radiation since the first galaxies formed over 13 billion years ago.
- Simply stated, light is energy. Light is the way energy travels through space.
- Whilst the term light can be used to refer to the whole electromagnetic spectrum, visible light refers to the small range of wavelengths our eyes are tuned to.
- The term light can be used in three different ways:
- Light can be used to mean the whole of the electromagnetic spectrum from radio waves, through visible light to gamma rays. When this meaning is intended, the terms radiant energy or photon energy are placed in brackets after the term light in this resource.
- Light can be used to mean the range of wavelengths and frequencies that can be detected by the human eye. A better term is visible light which refers to the wavelengths that correspond with the colours between red and violet, the visible spectrum.
- Light can also be used to mean the range of wavelengths and frequencies between infra-red and ultra-violet. This usage is sometimes useful because the outer limits of the visible spectrum can differ under different lighting conditions and for different individuals.
- Remember that the precise experience of visible light is not the same for all individual humans and is not the same for all living things.
- Light travels through a vacuum at 299,792,458 metres per second but propagates more slowly through other media.
- When light interacts with matter it results in optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
Optic radiation
The optic radiations are tracts formed from the axons of neurons located in the lateral geniculate nucleus and leading to areas within the primary visual cortex. There is an optic radiation on each side of the brain. They carry visual information through lower and upper divisions to their corresponding cerebral hemisphere.
Lateral geniculate nucleus
The lateral geniculate nucleus is a relay centre on the visual pathway from the eyeball to the brain. It receives sensory input from the retina via the axons of ganglion cells.
The thalamus which houses the lateral geniculate nucleus is a small structure within the brain, located just above the brain stem between the cerebral cortex and the midbrain with extensive nerve connections to both.
The lateral geniculate nucleus is the central connection for the optic nerve to the occipital lobe of the brain, particularly the primary visual cortex.
Both the left and right hemispheres of the brain have a lateral geniculate nucleus.
There are three major cell types in the lateral geniculate nucleus which connect to three distinct types of ganglion cells:
- P ganglion cells send axons to the parvocellular layer of the lateral geniculate nucleus.
- M ganglion cells send axons to the magnocellular layer.
- K ganglion cells send axons to a koniocellular layer.
The lateral geniculate nucleus specialises in calculations based on the information it receives from both the eyes and from the brain. Calculations include resolving temporal and spatial correlations between different inputs. This means that things can be organised in terms of the sequence of events over time and the spatial relationship of things within the overall field of view.
Some of the correlations deal with signals received from one eye but not the other. Some deal with the left and right semi-fields of view captured by both eyes. As a result, they help to produce a three-dimensional representation of the field of view of an observer.
- The outputs of the lateral geniculate nucleus serve several functions. Some are directed towards the eyes, others are directed towards the brain.
- A signal is provided to control the vergence of the two eyes so they converge at the principal plane of interest in object-space at any particular moment.
- Computations within the lateral geniculate nucleus determine the position of every major element in object-space relative to the observer. The motion of the eyes enables a larger stereoscopic mapping of the visual field to be achieved.
- A tag is provided for each major element in the central field of view of object-space. The accumulated tags are attached to the features in the merged visual fields and are forwarded to the primary visual cortex.
- Another tag is provided for each major element in the visual field describing the velocity of the major elements based on changes in position over time. The velocity tags (particularly those associated with the peripheral field of view) are also used to determine the direction the organism is moving relative to object-space.
Optic nerve
The optic nerve is the cable–like grouping of nerve fibres formed from the axons of ganglion cells that transmit visual information towards the lateral geniculate nucleus.
The optic nerve contains around a million fibres and transports the continuous stream of data that arrives from rods, cones and interneurons (bipolar, amacrine cells). The optic nerve is a parallel communication cable that enables every fibre to represent distinct information about the presence of light in each region of the visual field.
Müller cells
Müller glia, or Müller cells, are a type of retinal cell that serve as support cells for neurons, as other types of glial cells do.
An important role of Müller cells is to funnel light to the rod and cone photoreceptors from the outer surface of the retina to where the photoreceptors are located.
Other functions include maintaining the structural and functional stability of retinal cells. They regulate the extracellular environment, remove debris, provide electrical insulation of the photoreceptors and other neurons, and mechanical support for the fabric of the retina.
- All glial cells (or simply glia), are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system.
- Müller cells are the most common type of glial cell found in the retina. While their cell bodies are located in the inner nuclear layer of the retina, they span the entire retina.
Optical density is a measure of how much a material resists and slows the transmission of light.
- The higher the optical density of a material, the slower light travels through it.
- The lower the optical density of a material, the faster light travels through it.
- A vacuum is not a medium and has zero optical density.
- Light travels through a vacuum at the maximum possible speed of light which is 299,792 kilometres per second.
- Optical density and refractive index are related properties.
- In general, materials with higher optical density tend to have higher refractive indices and vice versa.
- The greater the difference in refractive index between two materials, the more they will bend light when they come into contact.
A human observer is a person who engages in observation by watching things.
- In the presence of visible light, an observer perceives colour because the retina at the back of the human eye is sensitive to wavelengths of light that fall within the visible part of the electromagnetic spectrum.
- The visual experience of colour is associated with words such as red, blue, yellow, etc.
- The retina’s response to visible light can be described in terms of wavelength, frequency and brightness.
- Other properties of the world around us must be inferred from light patterns.
- An observation can take many forms such as:
- Watching an ocean sunset or the sky at night.
- Studying a baby’s face.
- Exploring something that can’t be seen by collecting data from an instrument or machine.
- Experimenting in a laboratory setting.
An object is a material thing that has mass and occupies space.
- An object, as a physical entity, can be defined by its mass and the space it occupies. Objects are characterized by properties such as size, shape, texture, and colour, all of which are perceptible through the senses.
- The perception of an object arises from the brain’s interpretation of sensory information, primarily from the light reflected or transmitted by the object. At a fundamental level, all objects are composed of atoms or molecules, which form the basic structure of matter. The arrangement and behaviour of these atoms and molecules determine the object’s physical and chemical properties.
- Light plays a critical role in how objects are seen. Different atoms and molecules interact with light based on their unique structures and how they combine to form compounds or mixtures. When light strikes an opaque object, its surface molecules largely determine how the light is absorbed, reflected, or scattered. By contrast, translucent and transparent objects allow light to pass through, leading to different optical effects.
- Additionally, surface finish influences how light interacts with an object. Smooth, polished surfaces reflect light in a more uniform manner, while rough, textured, or rippled surfaces cause light to scatter in multiple directions.
As light crosses the boundary between two transparent media, the law of refraction (Snell’s law) states the relationship between the angle of incidence and angle of refraction of the light with reference to the refractive indices of both media as follows:
When electromagnetic radiation (light) of a specific frequency crosses the interface of any given pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.
- Snell’s law deals with the fact that for an incident ray approaching the boundary of two media, the sine of the angle of incidence multiplied by the index of refraction of the first medium is equal to the sine of the angle of refraction multiplied by the index of refraction of the second medium.
- Snell’s law deals with the fact that the sine of the angle of incidence to the sine of the angle of refraction is constant when a light ray passes across the boundary from one medium to another.
- Snell’s law can be used to calculate the angle of incidence or refraction associated with the use of lenses, prisms and other everyday materials.
- When using Snell’s law:
- The angles of incidence and refraction are measured between the direction of a ray of light and the normal – where the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
- The wavelength of the incident light is accounted for.
- The refractive indices used are selected for the pair of media concerned.
- The speed of light is expressed in metres per second (m/s).
A light source is a natural or man-made object that emits one or more wavelengths of light.
- The Sun is the most important light source in our lives and emits every wavelength of light in the visible spectrum.
- Celestial sources of light include other stars, comets and meteors.
- Other natural sources of light include lightning, volcanoes and forest fires.
- There are also bio-luminescent light sources including some species of fish and insects as well as types of bacteria and algae.
- Man-made light sources of the most simple type include natural tars and resins, wax candles, lamps that burn oil, fats or paraffin and gas lamps.
- Modern man-made light sources include tungsten light sources. These are a type of incandescent source which means they radiate light when electricity is used to heat a filament inside a glass bulb.
- Halogen bulbs are more efficient and long-lasting versions of incandescent tungsten lamps and produce a very uniform bright light throughout the bulb’s lifetime.
- Fluorescent lights are non-incandescent sources of light. They generally work by passing electricity through a glass tube of gas such as mercury, neon, argon or xenon instead of a filament. These lamps are very efficient at emitting visible light, produce less waste heat, and typically last much longer than incandescent lamps.
- An LED (Light Emitting Diode) is an electroluminescent light source. It produces light by passing an electrical charge across the junction of a semiconductor.
- Made-made lights can emit a single wavelength, bands of wavelengths or combinations of wavelengths.
- An LED light typically emits a single colour of light which is composed of a very narrow range of wavelengths.
An oscillation is a periodic motion that repeats itself in a regular cycle.
- Oscillation is a characteristic of waves, including electromagnetic waves.
- Examples of oscillation include the side-to-side swing of a pendulum and the up-and-down motion of a spring with a weight attached.
- Electromagnetic waves oscillate due to the transmission of energy by their electric and magnetic fields.
- An oscillating movement is typically around a point of equilibrium and the motion repeats itself around an equilibrium position.