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.
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.
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.
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.
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.