The joule (J) is the unit of energy, work, and heat in the International System of Units (SI).
- One joule is equal to the amount of work done when a force of one newton displaces an object by one meter in the direction of that force.
- It can also be defined as the amount of energy dissipated as heat when an electric current of one ampere flows through a resistance of one ohm for one second.
- While joules are a fundamental unit, they are a relatively small unit of energy. Therefore, larger units like kilojoules (kJ) or megajoules (MJ) are often used for practical applications.
Interneurons are a type of neuron found in the nervous system of animals, including humans, which play a role in processing and communicating information.
- Interneurons can be classified into different types based on their functions, such as local circuit interneurons and relay interneurons.
- Local circuit interneurons have short axons and form circuits with nearby neurons to analyse and process information locally.
- Relay interneurons have long axons and connect circuits of neurons in different regions of the central nervous system, enabling communication and integration of information.
- Interneurons can be further classified into sub-classes based on their neurotransmitter type, morphology, and connectivity.
- Interneurons serve as nodes within neural circuits, enabling communication and integration of sensory and motor information between the peripheral nervous system and the central nervous system.
Illuminance refers to the amount of light from a natural or artificial light source that falls on a surface. It is usually used to describe the usable light, regardless of the total brightness of the light source.
- Illuminance refers to the amount of light from a natural or artificial light source that falls on a surface. It is usually used to describe the usable light, regardless of the total brightness of the light source.
- When a book is placed on a table, different levels of illuminance can be observed depending on whether the sky is overcast, the time of day, or whether the surface is indirectly lit.
- Illuminance is a measure of the amount of light that falls on a surface per unit area. It is determined by the intensity of the light source and the distance from the light source to the surface, but is independent of the characteristics of the surface it strikes, such as its colour or reflectivity.
Illumination (lighting) is the deliberate use of light to achieve a practical, aesthetic or physiological effect.
- Illumination (lighting) is the deliberate use of light to achieve a practical, aesthetic or physiological effect.
- Illumination can be provided through artificial light sources such as lamps and light fixtures, or natural illumination by capturing daylight.
- Daylighting, which involves the use of windows, skylights or light shelves, is sometimes used as the main source of light during daytime in buildings.
- Specialized forms of artificial lighting have been developed to suit every possible situation and purpose where natural light is not available, such as in underground spaces or during nighttime.
- The colour temperature of light can affect how colours appear to an observer. Lighting producing a colour temperature below 4000K produces the impression of warmer colours. Lighting producing a colour temperature above 5000K produces the impression of cooler colours.
Incident light refers to light that is travelling towards an object or medium.
- Incident light refers to light that is travelling towards an object or medium.
- Incident light may come from the Sun, an artificial source or may have already been reflected off another surface, such as a mirror.
- When incident light strikes a surface or object, it may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical effects.
- Incident light is typically represented on a ray diagram as a straight line with an arrow to indicate its direction of propagation.
Intensity measures the amount of light energy passing through a unit area perpendicular to the direction of light propagation.
- Intensity measures the amount of energy carried by a light wave or stream of photons.
- When light is modelled as a wave, intensity is proportional to the square of the amplitude.
- When light is modelled as a particle, intensity is proportional to the number of photons present at any given point in time.
- The intensity of light falls off as the inverse square of the distance from a point light source increases.
- Light intensity at any given distance from a light source is directly related to the power of the light source and the distance from the source.
- The power of a light source describes the rate at which energy is emitted and is measured in watts.
- The intensity of light is measured in watts per square meter (W/m²) and is also commonly expressed in lux (lx).
Light interference occurs when two or more light waves interact with one another, resulting in a combination of their amplitudes. The resulting wave may increase or decrease in strength.
- A simple form of interference takes place when two plane waves of the same frequency meet at an angle and combine.
- Light interference is often observed as interference patterns, such as seen in supernumerary rainbows.
- Interference patterns are produced when the energy of waves combines constructively or destructively. For example, waves on a pond can create interference patterns.
- Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The resulting wave is the sum of the amplitudes of the original waves.
- Destructive interference occurs when the trough of one wave meets the crest of another wave. The resulting wave is the difference between the amplitudes of the original waves.
The refractive index (index of refraction) of a medium measures how much the speed of light is reduced when it passes through a medium compared to its speed in a vacuum.
- Refractive index (or, index of refraction) is a measurement of how much the speed of light is reduced when it passes through a medium compared to the speed of light in a vacuum.
- The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
- The refractive index can vary with the wavelength of the light being refracted. This phenomenon is called dispersion, and it is what causes white light to split into its constituent colours when it passes through a prism.
- The refractive index of a material can be affected by various factors such as temperature, pressure, and density.
Hue is one of the three main properties of colour, alongside saturation and brightness and is described using names such as red, yellow, green or blue.
- Hue refers to the colour of an object or light source, and is determined by the dominant wavelength of light it emits or reflects.
- Hue is often used to describe colours in terms of their position on the colour wheel. Colour wheels are circular diagrams that arrange colours according to their hue.
- The most commonly used colour wheel is the RGB colour wheel, which includes primary colours (hues) of red, green and blue, as well as secondary and tertiary colours.
- Hues can be warm or cool, depending on their position on the colour wheel. Warm hues are those that include red, orange and yellow, while cool hues include blue, green and purple.
- The perceived brightness and saturation of a hue can be affected by its surrounding colours, as well as by lighting conditions.
- The perception of hue is also influenced by cultural and personal associations, as well as context and other environmental factors.
The HSL colour model is similar to the HSB model. HSL refer to adjustments that can be made to hue, saturation and lightness to produce different colours. HSB refer to adjustments that can be made to hue, saturation and brightness to produce different colours.
- The HSL and HSB are very similar models and are often used interchangeably. They both represent colours based on Hue, Saturation, and a third component.
- In the HSB colour model brightness refers to the overall luminance of a colour.
- In the HSB colour model, brightness represents the colour independent of adding white or black. It’s like a dimmer switch for the chosen colour.
- In the HSL colour model, lightness refers to how light or dark a colour appears, considering how our eyes perceive brightness relative to a neutral grey. This is why a 50% light value represents a medium tone, even though it might not be the same brightness for all hues.
- Both the HSB colour model and the HSL colour model are usually represented as a cylinder, where the hue is represented by an angle around the central axis, the saturation is represented as the distance from the central axis, and the brightness or lightness is represented as a distance along the vertical axis.
- The difference between the two models lies in how the lightness and brightness components are calculated. In HSB, the brightness value is calculated by summing the highest and lowest RGB components and then dividing by two. In HSL, the lightness value is calculated by averaging the highest and lowest RGB components.
HSB colour values (codes) are numeric triplets used in software applications and programming to identify different colours.
- A numeric triplet is a code containing three parameters that refer to the hue, saturation, and brightness of a colour.
- For example:
- The HSB values for pure red are(0, 100%, 100%): Hue: 0°, Saturation: 100%, Brightness: 100%.
- A lighter, pastel version of red might be (0, 50%, 100%): Hue: 0°, Saturation: 50%, Brightness: 100%.
- A very dark, muted red could be: Hue (0, 100%, 20%): 0°, Saturation: 100%, Brightness: 20%.
- The values assigned to the three parameters can be used to define millions of different colours.
- Typically, the HSB colour model is implemented as follows:
- Hue is represented in degrees from 0 to 360, corresponding to locations on the circumference of a colour wheel.
- Saturation is represented as a percentage, where 100% denotes a fully saturated colour, and 0% denotes a fully desaturated colour.
- Brightness is represented as a percentage, where 100% denotes the highest luminance of a colour, and 0% denotes the darkest possible shade of a colour.
The HSB colour model is similar to the RGB colour model insofar as it is an additive model based on RGB primary colours.
- Both RGB and HSB are additive colour models with red, green and blue primary colours. But whilst RGB relies on directly adjusting the amount of red, green and blue light needed to produce other colours the HSB colour model relies on adjusting hue, saturation and brightness.
- Hue refers to the perceived difference between colours and is usually described using names such as red, yellow, green, or blue.
- Hue can be measured as a location on an HSB colour wheel and expressed as a degree between 0 and 360.
- Saturation refers to the vividness of a colour compared to an unsaturated colour.
- Saturation is measured between a fully saturated colour (100%) and an unsaturated colour (0%)that appears either:
- Dull and washed out until all colour disappears, leaving only a monochromatic grey tone (0%).
- Misty or milky the nearer they are to white.
- On many HSB colour wheels, saturation decreases from the edge to the centre.
- Brightness refers to the perceived difference in the appearance of colours under ideal sunlit conditions compared to poor lighting conditions where a hue’s vitality is lost.
- Brightness can be measured as a percentage from 100% to 0%.
- As the brightness of a fully saturated hue decreases, it appears progressively darker and achromatic.
Horizontal cells are neurons that interconnect with other types of neurons within the retina of the human eye.
- Horizontal cells are one of several types of neurons found in the retina of the human eye. The other types include photoreceptor cells (rods and cones), bipolar cells, amacrine cells, and ganglion cells.
- Horizontal cells interconnect with rod and cone cells via synapses, which is why they are often referred to as laterally interconnecting neurons.
- Horizontal cells help to integrate and regulate photoreceptor cells, cleaning up and globally adjusting signals passing through bipolar cells toward the region containing ganglion cells.
- An important function of horizontal cells is enabling the eye to adjust to both bright and dim light conditions. They achieve this by providing feedback to rod and cone photoreceptors about the average level of illumination falling onto specific regions of the retina.
- Horizontal cells are believed to prevent signals representing the brightest objects in a scene from dazzling the retina and degrading the quality of information.
Hexadecimal notation is a system for representing RGB colours. For example, a computer display would use the code #FF0000 to produce a bright red pixel. It is commonly used in digital applications such as web design and image processing, allowing for the accurate specification of up to 16,777,216 different colours.
- In hexadecimal notation, each of the three RGB colour components—red, green, and blue—is assigned a value between 00 and FF, where 00 represents no intensity and FF represents maximum intensity.
- For example:
- Red can have a value from 00 to FF (e.g., 00).
- Green is also assigned a value between 00 and FF (e.g., 0F).
- Blue follows the same pattern (e.g., FF).
- These three values form a six-digit hexadecimal triplet. For instance, the values above combine to form #000FFF, where the hash symbol (#) indicates hexadecimal notation.
- Some common colours and their hexadecimal representations are:
- Red (#FF0000)
- Yellow (#FFFF00)
- Green (#00FF00)
- Cyan (#00FFFF)
- Blue (#0000FF)
- Magenta (#FF00FF
In the context of images, a greyscale colour model represents a picture using only shades of grey, from pure black to pure white. There’s no colour information included. This is commonly used in black-and-white photography or to convert colour images into black and white.
- The greyscale colour model is used for:
- Converting colour images to black-and-white.
- Creating black-and-white images through cameras, scanners, and other input devices.
- Three algorithms are commonly used for greyscale conversion: the lightness method, the weighted average method, and the luminosity method.
- The greyscale colour model is not a simple linear scale from black to white but rather a method of converting colour brightness to reflect tonal relationships. When converting digital images to greyscale, each pixel is assigned a corresponding level of brightness based on its colour.
- When fully saturated spectral colours are converted to greyscale, their brightness typically ranges between 11% and 89%. For example:
- Red = 70%
- Orange = 40.38%
- Yellow = 11%
- Green = 41%
- Blue = 89%
- Violet = 74.06%
- Any RGB decimal colour value can be converted to greyscale. For instance, the RGB value for cyan converts to a greyscale value of 178, 178, 178. Similarly, HSB colour values can also be converted to greyscale, with the HSB value for pure yellow being Hue = 0, Saturation = 0, and Brightness = 11%.
The gravitational force, also called gravity, is one of the four fundamental forces in nature. The other forces are the electromagnetic force, the strong nuclear force and the weak nuclear force.
- Gravity is the phenomenon that attracts objects with mass or energy towards one another.
- It affects celestial bodies such as planets, stars, galaxies, and even light.
- The influence of gravity on smaller objects like human beings in the presence of larger ones, such as planets, is evident.
- Gravity, such as the Moon’s gravity, leads to ocean tides on Earth.
- Gravity accounts for the weight of physical objects. Its range is infinite, although its effects weaken as objects move farther apart
- Gravitational force is a universal force, meaning that it acts between all objects with mass, regardless of their composition or charge.
- Gravitational force is a long-range force, meaning that it can act between objects that are very far apart.
A raindrop is often represented as a geometrically perfect sphere, an idealized form that rarely exists in reality. This simplification helps in understanding the physics of rainbows, even though actual raindrops seldom maintain a perfectly spherical shape.
- Although the idealized geometry of raindrops aids in understanding rainbows, real raindrops vary in shape due to factors such as size, speed of descent, and turbulence. Each rainbow we observe is unique, shaped by chance and a range of environmental conditions.
- In summary, the form of a rainbow and the arrangement of raindrops within it depend on various changing factors, including the size, shape, and distribution of the droplets, the position of the sun, the observer’s location, atmospheric clarity and composition, and the presence of additional light sources or reflective surfaces. Every rainbow is a one-of-a-kind phenomenon, shaped by both random variations and the surrounding environment.
A retinal ganglion cell is a type of neuron located in the retina of the human eye. It receives visual information from photoreceptors through two types of intermediate neurons: bipolar cells and amacrine cells. Retinal ganglion cells transmit both image-forming and non-image-forming visual information to several regions in the brain, including the thalamus, hypothalamus, and midbrain.
- Retinal ganglion cells are located near the boundary between the retina and the central chamber containing the vitreous humour. They collect and process visual information from around forty different types of cells, including rods, cones, bipolar, horizontal, and amacrine cells. Once processed, this information is transmitted via their axons to higher visual centres in the brain
- The axons of ganglion cells form the fibres of the optic nerve, which synapse onto the lateral geniculate nucleus. Axons are long, slender projections of the cell body that typically conduct electrical impulses, known as action potentials, away from the neuron.
- In the fovea at the centre of the macula, a single ganglion cell communicates with as few as five photoreceptors, producing the highest possible resolution of detail. At the retina’s extreme periphery, however, a single ganglion cell receives input from thousands of photoreceptors.
- There are approximately twenty functional types of ganglion cells, which resolve visual information from 120 million rods and cones into a million parallel streams. These cells complete the foundation of visual processing in the retina, encoding the eye’s response to light and forming the fundamental building blocks of vision. Ganglion cells enable this encoding to converge into a unified representation of the visual world, creating the basis for human visual experience.
The term gamut or colour gamut is used to describe:
- The range of colours that a specific device or system can display or reproduce.
- The range of colours that the human eye can see in specific conditions.
- A range of colours that is smaller than all the colours that the human eye can see.
- All the colours in an image. Digitizing a photo, changing an image’s colour space, or printing an image onto paper might change its gamut.
- The range of perceived colours (visible to a human observer) is always greater than the range that can be reproduced by any digital device such as a screen, monitor or projector.
- Digital cameras, scanners, monitors, and printers all have limits to the range of colours they can capture, save, and reproduce.
- The main use of digital colour spaces and colour profiles is to set the gamut of colours that can be used to accurately reproduce or optimise the appearance of an image.
- It is currently impossible to make a digital device that can reproduce the same range of colours that the human eye can see.
Gamma correction, also referred to as gamma encoding, is an image processing technique that adjusts the brightness and contrast of an image to achieve a more natural and visually pleasing appearance.
- Gamma correction of digital images prevents excessive storage of information about highlights that are invisible to humans and ensures sufficient information is retained for shadows that require more differentiation to be observed.
- Gamma correction adjusts the relationship between the numerical value of a pixel stored in an image file (e.g., JPG or TIFF) and its corresponding brightness when displayed on-screen.
- Gamma correction is typically performed to compensate for the non-linear relationship between the input signal and the displayed brightness on a monitor or screen.
- In the case of a black-and-white image, a gamma function impacts highlights (brightest values), mid-tones (greyscale), and shadows (dark areas) in distinct ways.
- Gamma correction is not limited to black and white images but applies to colour images, where it affects colour balance and contrast.
A free electron is an electron that is no longer bound to a specific atom, allowing it to move freely within a material.
- Photoelectric Effect: Free electrons are involved in the photoelectric effect, where photons (light particles) strike a material and transfer energy to electrons. If the energy from the light is sufficient, it can release electrons from their bound state, creating free electrons. This phenomenon is fundamental to the operation of devices like solar cells and photodetectors.
- Interaction with Light: Free electrons can scatter light. When light interacts with a material, free electrons can absorb and re-emit photons, contributing to effects like reflection, refraction, and the generation of certain colours in materials.
- Plasma and Light: In a plasma state, which consists of free electrons and ions, light behaves differently compared to its behaviour in neutral gases. Free electrons can reflect and absorb electromagnetic radiation, influencing how light propagates through plasma.
- Electrical Conductivity and Light Emission: In conductors, free electrons facilitate electrical currents, and when these electrons transition between energy levels, they can emit light, as seen in incandescent bulbs or LED technology.
In physics, fundamental forces cannot be explained through simpler or more elementary interactions, so are regarded as fundamental building blocks of the natural world.
The four fundamental forces that account for all the forms of pulling and pushing between things are:
Electromagnetic force
- The electromagnetic force is the interaction that arises between electrically charged particles, such as electrons and is characterized by positive or negative charges. Oppositely charged particles exert an attractive force, while particles with the same charge exert a repulsive force. Photons carry electromagnetic force through electric and magnetic fields, propagating at the speed of light.
Weak Nuclear force
- In nuclear physics and particle physics, the weak nuclear force mediates interactions between subatomic particles and is responsible for radioactive decay in atoms. The weak nuclear force doesn’t affect electromagnetic radiation.
Strong Nuclear force
- The strong nuclear force binds matter together and is responsible for holding together protons and neutrons which are the subatomic particles within the atomic nucleus. It counteracts repulsive electromagnetic forces that push subatomic particles apart but only operate over the smallest imaginable distances. The strong nuclear force plays a central role in storing the energy that is used in nuclear power and nuclear weapons.
Gravitational force
- Gravity is the phenomenon that attracts objects with mass or energy towards one another. It affects celestial bodies such as planets, stars, galaxies, and even light. The influence of gravity on smaller objects like human beings in the presence of larger ones, such as planets, is evident. Gravity, such as the Moon’s gravity, leads to ocean tides on Earth. Gravity accounts for the weight of physical objects. Its range is infinite, although its effects weaken as objects move farther apart.
The fovea centralis is the region of the eye that provides the optimal location for forming detailed images.
- The eyes continuously rotate in their sockets to focus objects of interest as precisely as possible onto the fovea centralis.
- These rapid movements, called saccades, position objects of interest on the fovea, allowing for detailed inspection of the environment.
- Although the entire surface of the retina contains nerve cells, the fovea is the small region (about 0.25 mm in diameter) at the centre of the macula which has the highest concentration of cones, making it ideal for capturing fine detail.
- While cones are concentrated in the fovea for detecting fine detail and colour, rods, which are more sensitive to light but not colour, are spread throughout the rest of the retina and are essential for peripheral and low-light vision.
Each fundamental force is conveyed by a distinct particle type known as a force carrier. These carriers are responsible for transmitting forces between pairs of particles.
- Take light as an example of a force carrier for electromagnetic radiation.
- Light is a form of energy that travels as waves, but it also behaves like a stream of tiny particles called photons.
- These photons are the force carriers for the electromagnetic force, one of the fundamental forces in the universe.
- The electromagnetic force is responsible for a variety of phenomena, including the attraction between oppositely charged particles and the repulsion between like charges.
- Photons can also interact with individual electrons in atoms, causing them to move or change energy levels.
- Follow this link to find out more about fundamental forces.
In physics, a force is anything that can make an object move differently. It’s like a push or a pull that can make an object start moving, stop moving, or change direction. Imagine kicking a soccer ball – the kick is the force that makes the ball move.
- Forces can be either contact forces or non-contact forces.
- Contact forces: These happen when two objects touch, like friction when you rub your hands together, or the push you give the ball.
- Non-contact forces: These act even when objects aren’t touching, like gravity pulling you down, or a magnet attracting a paperclip.
- Non-contact forces are forces that act between objects that are not in contact with each other. Examples of non-contact forces include gravity, electromagnetism, and the strong nuclear force.
- Forces can make things move faster (accelerate), slower (decelerate), or change direction altogether.
- Objects, bodies, matter, particles, radiation, and space-time are all in motion.
- On a cosmological-scale, concentrated matter in planets, stars, and galaxies leads to significant push-pull interactions.
- Motion signifies a change in the position of the elements of a physical system including translational motion, rotational motion, vibrational motion, and oscillatory motion.
Fluorescence is a type of luminescence, a light source resulting from the temporary absorption and emission of electromagnetic radiation by certain materials.
- Fluorescence occurs when these materials “catch” light of a specific colour and then quickly “re-emit” it as a different, usually lower-energy (longer wavelength) colour.
- Unlike light sources that involve flames or extreme heat, fluorescence happens through a rapid physical process in the material itself.
An electromagnetic field (which includes both electric and magnetic fields) is the region around an object where it can exert a force on another object without direct contact. Electric fields arise from charged objects, while magnetic fields are produced by moving charges, such as electric currents.
- Fields are fundamental in physics, playing key roles in areas like electromagnetism, quantum mechanics, and general relativity.
- Fields can be represented by lines showing the direction of the force experienced by objects within the field.
- Fields are created by a source object and influence other objects within their range.
- Electromagnetic fields combine electric and magnetic components, interconnected through electromagnetic waves.
- Electric fields are associated with positive or negative charges and exert forces on charged objects.
- Magnetic fields are generated by moving electric charges, such as currents in wires, and can affect magnetic materials and charged particles.
- Electric fields and magnetic fields together make up the electromagnetic field, which governs interactions between charged particles.
- According to quantum field theory, all particles and forces in the universe arise from interactions between underlying fields, which give rise to the properties of matter and energy.
Ganglion cells
Retinal ganglion cells are located near the boundary between the retina and the central chamber containing vitreous humour. They collect and process all the visual information gathered directly or indirectly from the forty-something types of rod, cone, bipolar, horizontal and amacrine cells and, once finished, transmit it via their axons towards higher visual centres within the brain.
The axons of ganglion cells form into the fibres of the optic nerve that synapse at the other end on the lateral geniculate nucleus. Axons take the form of long slender fibre-like projections of the cell body and typically conduct electrical impulses, often called action potentials, away from a neuron.
A single ganglion cell communicates with as few as five photoreceptors in the fovea at the centre of the macula. This produces images containing the maximum possible resolution of detail. At the extreme periphery of the retina, a single ganglion cell receives information from many thousands of photoreceptors.
Around twenty distinguishable functional types of ganglion cells resolve the information received from 120 million rods and cones into one million parallel streams of information about the world surveyed by a human observer in real-time throughout every day of their lives. They function to complete the construction of the foundations of visual experience by the retina, ordering the eyes response to light into the fundamental building blocks of vision. Ganglion cells do the groundwork that enables retinal encodings to ultimately converge into a unified representation of the visual world.
Ganglion cells not only deal with colour information streaming in from rod and cone cells but also with the deductions, inferences, anticipatory functions and modifications suggested by bipolar, amacrine and horizontal cells. Their challenge, therefore, is to enable all this data to converge and to assemble it into high fidelity, redundancy-free, compressed and coded form that can continue to be handled within the available bandwidth and so the data-carrying capacity of the optic nerve.
It is not hard to imagine the kind of challenges they must deal with:
- Information must feed into and support the distinct attributes of visual perception and be available to be resolved within the composition of our immediately present visual impressions whenever needed.
- Information must correspond with the outstanding discriminatory capacities that enable the visual system to operate a palette that can include millions of perceivable variations in colour.
- Information about the outside world must be able to be automatically cross-referenced, highly detailed, specifically relevant, spatial and temporally sequenced and available on demand.
- Information must be subjectively orientated in a way that it is locked at an impeccable level of accurate detail to even our most insane intentions as we leap from rock to rock across a swollen river or dive from an aircraft wearing only a wingsuit and negotiate the topography of a mountainous landscape speeding past at 260km per hour.
It is now known that efficient transmission of colour information is achieved by a transformation of the initial three trivariant colour mechanisms of rods and cones into one achromatic and two chromatic channels. But another processing stage has now been recognised that dynamically readjusts the eye’s trivariant responses to meet criteria of efficient colour information management and to provide us with all the necessary contextualising details as we survey the world around us. Discussion of opponent-processing is dealt with in the next article!
Horizontal cells
Horizontal cells are connected to rod and cone cells by synapses and are classed as laterally interconnecting neurons.
Horizontal cells help to integrate and regulate information received from photoreceptor cells, cleaning up and globally adjusting signals passing through bipolar cells towards the regions containing ganglion cells.
An important function of horizontal cells is enabling the eye to adjust to both bright and dim light conditions. They achieve this by providing feedback to rod and cone photoreceptors about the average level of illumination falling onto specific regions of the retina.
If a scene contains objects that are much brighter than others, then horizontal cells are believed to prevent signals representing the brightest objects from dazzling the retina and degrading the overall quality of information.
Caption
The Neuronal Organization of the Retina Richard H. Masland
https://www.cell.com/neuron/fulltext/S0896-6273(12)00883-5?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0896627312008835%3Fshowall%3Dtrue
Fovea centralis
The entire surface of the retina contains nerve cells, but there is a small portion with a diameter of approximately 0.25 mm at the centre of the macula called the fovea centralis where the concentration of cones is greatest. This region is the optimal location for the formation of image detail. The eyes constantly rotate in their sockets to focus images of objects of interest as precisely as possible at this location.
Internal reflection occurs when light travelling through a medium, such as water or glass, reaches the boundary with another medium, like air, and a portion of the light reflects back into the original medium. This happens regardless of the angle of incidence, as long as the light encounters the boundary between the two media.
- Internal reflection is a common phenomenon not only for visible light but for all types of electromagnetic radiation. For internal reflection to occur, the refractive index of the second medium must be lower than that of the first medium. This means internal reflection happens when light moves from a denser medium, such as water or glass, to a less dense medium, like air, but not when light moves from air to glass or water.
- In everyday situations, light is typically both refracted and reflected at the boundary between water or glass and air, often due to irregularities on the surface. If the angle at which light strikes this boundary is less than the critical angle, the light is refracted as it crosses into the second medium.
- When light strikes the boundary exactly at the critical angle, it neither fully reflects nor refracts but travels along the boundary between the two media. However, if the angle of incidence exceeds the critical angle, the light will undergo total internal reflection, meaning no light passes through, and all of it is reflected back into the original medium.
- The critical angle is the specific angle of incidence, measured with respect to the normal (a line perpendicular to the boundary), above which total internal reflection occurs.
- In ray diagrams, the normal is an imaginary line drawn perpendicular to the boundary between two media, and the angle of refraction is measured between the refracted ray and the normal. If the boundary is curved, the normal is drawn perpendicular to the curve at the point of incidence.
Light travels at different speeds through various media, such as air, glass, or water. A “fast” medium is one where light moves more quickly compared to other materials.
- In a vacuum, light travels at 299,792 kilometres per second, but in other media, it slows down.
- In some cases, the speed is close to that in a vacuum, while in others, it is significantly slower.
- Knowing whether a medium is fast or slow helps predict how light will behave when it crosses from one medium to another.
- If light crosses from a fast medium to a slower one, it bends towards the normal.
- If light crosses from a slow medium to a faster one, it bends away from the normal.
- In optics, the normal is a line drawn perpendicular (at a 90° angle) to the boundary between two media in a ray diagram.
The hertz (symbol: Hz) is a unit used to measure the frequency of electromagnetic waves. It represents the number of wave-cycles per second.
- One hertz is defined as one cycle per second.
- Hertz measure the number of oscillations of the perpendicular electric and magnetic fields in electromagnetic radiation per second.
- Frequency conversions:
- 1 Hertz (Hz) = 1 cycle per second
- 1 Kilohertz (kHz) = 1,000 (thousand) cycles per second
- 1 Megahertz (MHz) = 1,000,000 (million) cycles per second
- 1 Gigahertz (GHz) = 1,000,000,000 (billion) cycles per second
- 1 Terahertz (THz) = 1,000,000,000,000 (trillion )cycles per second
The frequency of electromagnetic radiation (light) refers to the number of wave-cycles of an electromagnetic wave that pass a given point in a given amount of time.
- Frequency is measured in Hertz (Hz) and signifies the number of wave-cycles per second. Sub-units of Hertz enable measurements involving a higher count of wave-cycles within a single second.
- The frequency of electromagnetic radiation spans a broad range, from radio waves with low frequencies to gamma rays with high frequencies.
- The wavelength and frequency of light are closely linked. Specifically, as the wavelength becomes shorter, the frequency increases correspondingly.
- It is important not to confuse the frequency of a wave with the speed at which the wave travels or the distance it covers.
- The energy carried by a light wave intensifies as its oscillations increase in number and its wavelength shortens.