## Kinetic energy

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.
• When one object strikes another, the force of the impact can transfer kinetic energy.
• 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.
• Energy is the capacity to do work. It can exist in different forms such as kinetic, potential, thermal, and electromagnetic.
• Energy cannot be created or destroyed; it can only be converted from one form to another
• Force is a term used to describe the interaction between objects that causes a change in motion or shape.
• Galaxies, human beings and electrons can all apply forces to the things around them.
• Work is defined as the product of the force applied on an object and the distance it moves in the direction of the force.
• Objects at absolute zero do not have any thermal energy to transfer, but they can still interact and exchange other forms of energy.
##### At an atomic scale
• Electrical energy passes through a circuit as electrons flow, transferring their kinetic energy to other electrons in the circuit.
• Heat is produced as photons strike an object, transferring their energy to electrons within the atoms or molecules of its surface.
##### At a human scale
• The human senses of sight, hearing, and touch are tuned to respond to different forms of energy when a force is applied to them.
• When a person hears a sound or sees something, that is evidence of energy having been transferred to their senses.
##### Here is an example
• A person pushes a heavy boulder up a hill.
• Each time they push they apply force to make the boulder move.
• They feel exhausted by the time they reach the top because of the work involved in overcoming the force of gravity and friction.
• But the energy is not lost, instead, it is transferred to the boulder as kinetic energy.
• As soon as the boulder is released and starts to roll back down the hills, the kinetic energy it has gained is transferred to other boulders it crashes into.
• If the boulder is too heavy to move, no work is done on the boulder. However, the person’s muscles still expend energy, which is released into the environment as heat.

## lateral geniculate nucleus

###### 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.

## Lateral geniculate nucleus

The lateral geniculate nucleus is a relay center in the visual pathway from the eye to the brain. It receives a major 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 and has extensive nerve connections to both.
• The lateral geniculate nucleus specializes in processing information from both eyes and the brain. It resolves temporal and spatial correlations between different inputs, which helps organize the sequence of events over time and the relationship of objects within the visual field.
• Some correlations deal with signals received from one eye, while others deal with the left and right visual fields captured by both eyes. This helps produce a three-dimensional representation of the visual field.
• The lateral geniculate nucleus is the central connection for the optic nerve to the primary visual cortex of the occipital lobe.
• Both the left and right hemispheres of the brain contain 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 cells send axons to the parvocellular layer of the lateral geniculate nucleus.
• M cells send axons to the magnocellular layer.
• K cells send axons to the koniocellular layer.

## Law of refraction

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).

## Law of refraction

The law of refraction is a basic law of optics that governs the behaviour of light as it passes through a boundary between two different media, such as air and water.

The law of refraction states that the ratio of the sines of the angles of incidence and refraction of a light ray passing through the boundary of two media is constant, where the ratio is equal to the ratio of the refractive indices of the two media.

• The law of refraction (Snell’s law) deals with what happens to light as it crosses the boundary between two transparent media with different refractive indices.
• The law of refraction is useful because it relates the angle of incidence and angle of refraction of a ray of light to the refractive indices of the two media it is passing through.
• Transparent media all have different refractive indices (index of refraction) that measure how much the speed and direction of light changes as it passes out of one and into another.
• Factors that affect the refractive index of a medium include the wavelength of light passing through it, its optical density and its temperature.
• So the law of refraction explains the relationship between the angle of incidence as light approaches the boundary of one medium with a particular refractive index and the angle of refraction as it enters the second with a different refractive index.
• It derives a formula from the fact that when light of a particular frequency crosses the boundary between any 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.
• The formula is: n1 sin θ1 = n2 sin θ2, where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively.
• Refractive indices can be measured experimentally using techniques like refractometry, and they are unique to each medium and can be used to identify unknown substances.
• There are only four terms in the law of refraction so if three are known then the fourth can be calculated.
• So, for example, it is possible to calculate the angle of refraction associated with the use of a lens or prism if the angle of  incidence and the refractive indices of the first medium (air) and the second (optical glass) are known.
###### References

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).

## Laws of reflection

###### About the laws of reflection

When light reflects off a surface or object, it behaves predictably, following three laws known as the laws of reflection.

• The three laws of reflection are:
• The incident ray, the reflected ray, and the normal to the surface all lie within the same plane.
• The angle of incidence, which is the angle between the incident ray and the normal, is equal to the angle of reflection, which is the angle between the reflected ray and the normal.
•  The incident ray and the reflected ray are always on opposite sides of the normal, and all three lie within the same plane.

## Laws of refraction and reflection

The path of light through a raindrop is a key factor in determining whether it will direct light towards an observer and contribute to their perception of a rainbow. This can be broken down as follows:

• The impact parameter is a measure of the direction from which rays of incident light approach a raindrop and the point at which they strike the surface.
• When using a ray-tracing diagram to map the path of rays through a raindrop, an impact parameter scale is used to select which incident rays are of interest.
• An impact parameter scale is aligned with parallel incident rays and divides the relevant part of the surface of a droplet into equal parts.
• Using a scale with steps between zero and one, 0 is aligned with the ray that passes through the centre of a droplet and 1 with the ray that grazes the surface without refraction or reflection.
###### Remember that:
• Primary rainbows form when incident light strikes raindrops above their horizontal axis reflecting once off the inside before exiting towards an observer.
• Incident light that strikes raindrops below their horizontal axis and reflects once on the inside before exiting, directs light upwards away from an observer.
• Secondary rainbows form when incident light strikes raindrops below their horizontal axis reflecting twice off the inside before exiting downwards.
• The Law of reflection deals with the angles of incidence and reflection when light strikes and bounces back off a surface and can be used for calculations relating to the curved surfaces of a raindrop.
• Remember that the law of reflection states that the angle of incidence always equals the angle of reflection for a mirror-like (specular) surface.
• The Law of Refraction (Snell’s law) deals with the changes in the speed and direction of incident light as it crosses the boundaries between air and a raindrop and then between a raindrop and the surrounding air.
• An equation can be derived from Snell’s law that deals with the relationship between the angle of incidence and the angle of refraction of light with reference to the refractive indices of both media.

## LED

An LED is a type of electroluminescent light source that emits light when current flows through a semiconductor material.

• An LED typically emits light of a single colour that is composed of a narrow range of wavelengths.
• Multicoloured LEDs typically have three diodes that emit red, green, and blue light.
• By adjusting the relative brightness of the primary colours, a multicoloured LED can create a vast array of colours.
• By combining the three primary colours of red, green, and blue light in equal proportions multicoloured LEDs can produce white light.
• Typical peak wavelengths for multicoloured LEDs might be red = 625 nanometres, green = 500 nm, blue = 440 nm.
• LED light sources are commonly used to demonstrate the effects of projecting additive primary colours onto a dark surface.

## 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, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies 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 of the electromagnetic spectrum, visible light refers to the small range of wavelengths that 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.

## Light

Light is electromagnetic radiation that travels in waves or particles (photons) and can propagate through a vacuum as well as transparent and translucent media.

##### light in classical physics
• Classical physics thinks of light as continuous waves. This means that a wave of light in the vacuum of space has a constant wavelength (so colour), frequency and brightness and that it propagates through space without any reduction in force or energy.
• This Classical view is applied in the sub-field of optics when dealing with things like the reflection, refraction and polarisation of light and when analysing the behaviour of lenses, mirrors, and lasers. Visible light can be described in terms of:
• The electromagnetic wave theory of light is a key component of our understanding of visible light and its interactions with matter. It helps to explain phenomena such as reflection, refraction, and diffraction of light and plays a crucial role in technologies such as wireless communication, remote sensing and medical imaging.
##### light in quantum mechanics
• In the field of quantum mechanics, light is described as a stream of particles called photons, which are the quanta of the electromagnetic field. According to this theory, photons are massless particles of light that have no electric charge but have momentum and each photon constitutes a single packet of electromagnetic energy.
• One of the most famous experiments that demonstrate the particle-like nature of light is the photoelectric effect, in which electrons are emitted from a metal surface when exposed to certain wavelengths of light. The photoelectric effect can not be explained by the wave theory of light but is explained by Einstein’s theory of the photoelectric effect, which proposes that photons transfer their energy to electrons in the metal.
• The wave model and the quantum mechanical model of light are not mutually exclusive and can be used to develop different perspectives on the same phenomena.
• The wave model is useful for understanding light in situations where it behaves like a wave but largely ignores the way it interacts with matter at a sub-atomic scale.
• The quantum mechanical model of light is useful in understanding interactions between light and matter at a sub-atomic scale, particularly interactions involving single photons and other quantum particles such as electrons.
• Light and colour are related but distinct concepts. Light is a form of electromagnetic radiation, while colour is a perception that results from how the human eye and brain respond to different wavelengths of visible light.
• The human eye can perceive only a small part of the electromagnetic spectrum, known as visible light, which includes wavelengths between about 400 and 700 nanometres.
• The perception of colour depends on the wavelengths of light that stimulate the cones in the retina.
• The perception of colour can vary among individuals and living organisms.
• Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.
• Colour perception in humans primarily depends on the design of our eyes and the wavelength, frequency, and energy of the visible light that strikes the retina at the back of our eyes.
• Colour is a visual experience unique to each of us at any given moment because of our different points of view and perspectives on the world. So we share our experiences of colour using language to share our experiences of colour.

There is a difference in meaning between the terms light, electromagnetic radiation, radiant energy and electromagnetic energy in physics.

##### Light
• Light is best used to refer to the subset of electromagnetic radiation that is visible to the human eye, ranging from violet (shorter wavelengths) to red (longer wavelengths).
• Electromagnetic radiation refers to the transfer of all forms of electromagnetic radiation through space by electromagnetic waves and includes gamma rays, ultraviolet (UV), infrared (IR), X-rays, and radio waves, as well as visible light.
• Radiant energy is most commonly used to refer to electromagnetic radiation carried by electromagnetic waves. Radiant energy can be measured using instruments such as photometers, which detect the intensity of light or other forms of electromagnetic radiation.
##### Electromagnetic energy
• Electromagnetic energy is a more general term that refers to any form of energy that is carried by electromagnetic waves, including both radiant energy and other types of energy that are not radiant (e.g., static electric fields).
• The type of energy associated with electromagnetic radiation is a measurable quantity in physics, and its measurement is essential for understanding and analyzing physical systems and processes.
• The unit of measurement for electromagnetic energy in the International System of Units (SI) is the joule (J), which is defined as the amount of energy required to perform one joule of work
• The electronvolt (eV) is another unit of energy commonly used in atomic and subatomic physics.
###### References
• https://en.wikipedia.org/wiki/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, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies 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 of the electromagnetic spectrum, visible light refers to the small range of wavelengths that 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 exactly 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.

## Light & colour

• Light and colour are related but distinct concepts. Light is a form of electromagnetic radiation, while colour is a perception that results from how the human eye and brain respond to different wavelengths of visible light.
• The human eye can perceive only a small part of the electromagnetic spectrum, known as visible light, which includes wavelengths between about 400 and 700 nanometres.
• The perception of colour depends on the wavelengths of light that stimulate the cones in the retina.
• The perception of colour can vary among individuals and living organisms.
• Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.
• Colour perception in humans primarily depends on the design of our eyes and the wavelength, frequency, and energy of the visible light that strikes the retina at the back of our eyes.
• Colour is a visual experience unique to each of us at any given moment because of our different points of view and perspectives on the world. So we share our experiences of colour using language to share our experiences of colour.

## Light source

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.

## Light source

A light source is any object that emits electromagnetic radiation within the visible spectrum or other areas of the electromagnetic spectrum.

• The Sun is the most important natural light source for life on Earth and emits a broad spectrum of electromagnetic radiation, including visible light.
• Celestial sources of light include our own Sun and other stars, planets, moons, asteroids, comets, and meteors.
• Natural sources of light include lightning, volcanoes and forest fires.
• Bioluminescent organisms that produce light include some species of fish, fireflies, glow-worms, and certain types of fungi, bacteria and algae.
• Human-made light sources of the simplest types include natural tars and resins, wax candles, oil lamps, and gas lamps.
• Modern human-made light sources include tungsten filament incandescent bulbs, which produce light by heating a filament inside a glass bulb.
• Halogen bulbs are a type of incandescent lamp that uses a halogen gas to improve efficiency and lifespan, and produce a bright, uniform light.
• Fluorescent lamps are non-incandescent sources of light that work by exciting a gas or vapour with electricity, which produces ultraviolet light that is converted into visible light by a phosphorescent coating on the inside of the tube. These lamps are highly efficient and long-lasting.
• An LED is a type of semiconductor device that emits light when an electrical current passes through it.
###### References
• https://en.wikipedia.org/wiki/Light#Light_sources

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.

## Light sources for rainbows

The best light source for a rainbow is a strong point source such as sunlight. Sunlight is ideal because it is so intense and contains all the wavelengths that make up the visible spectrum.

• A human observer with binocular vision (two eyes) has a 1200 field of view from side to side. In clear conditions, the Sun can be considered to be a point-source filling just 0.50 of their horizontal field of view.
• A wide range of visible wavelengths of light is needed to produce all the rainbow colours. The Sun produces a continuous range of wavelengths across the entire visible spectrum.
• When atmospheric conditions like cloud or fog cause too much diffusion of sunlight before it strikes a curtain of rain, no bow is formed.
• Artificial light sources such as LED’s, incandescent light bulbs, fluorescent lights and halogen lamps all make poor light sources because they emit too narrow a range of wavelengths and don’t emit sufficient energy.

## Light stimulus

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.

## Light wave

A light wave is a type of electromagnetic radiation resulting from the interaction between oscillating electric and magnetic fields.

• Key features of electromagnetic waves are:
• Wavelength (λ): The wavelength of a light wave is the distance between two consecutive peaks or troughs of the wave. Wavelength determines the colour of the light, with shorter wavelengths appearing as blue or violet and longer wavelengths appearing as red or orange.
• Frequency (f): The frequency of a light wave is the number of wave cycles that pass a given point per second. Higher frequencies correspond to higher energy levels.
• Amplitude (A): The amplitude of a light wave refers to the height or intensity of the wave. The greater the amplitude, the brighter the light appears.
• Velocity (v): Velocity refers to the speed at which the wave travels through a medium, such as air or a vacuum. The velocity of light can be affected by the medium it travels through.
• Electromagnetic radiation is carried by an electromagnetic wave.
• Electromagnetic radiation is measured in terms of the amount of electromagnetic energy carried by an electromagnetic wave.
• Electromagnetic waves are synchronized oscillations of electric and magnetic fields that propagate at the speed of light in a vacuum.
• The energy carried by electromagnetic waves is often referred to as radiant energy.
• Electromagnetic radiation can also be described in terms of elementary particles called photons.
• We can feel electromagnetic waves release their energy when sunlight warms our skin.
• The position of an electromagnetic wave in the electromagnetic spectrum can be characterized by either its frequency of oscillation or wavelength.
• The electromagnetic spectrum includes, in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
• The limit for long wavelengths is the size of the observable universe which is estimated to be around 93 billion light-years in diameter.
• The short wavelength limit is still a topic of theoretical debate and research, and it is not yet definitively known whether there is a limit at the Planck length.

## Light, colour & vision

##### Light
• The human eye and human vision are adapted and responsive to the visible spectrum, which includes wavelengths of light corresponding to colours ranging from red to violet..
• Light is the electromagnetic radiation that enables us to perceive colour. It consists of a spectrum of wavelengths, and it is the interaction of these wavelengths with our visual system that gives rise to the perception of different colours.
• The visible spectrum is the range of wavelengths of light that the human eye can detect, typically spanning from approximately 400 nanometers (nm) for violet to 700 nm for red.
• Light is seldom composed of a single wavelength, so an observer is typically exposed to a range of diverse wavelengths or a combination of wavelengths from various parts of the visible spectrum.
• Visible light does not possess any properties that set it apart from other segments of the electromagnetic spectrum.
##### Colour
• Colour is not an inherent property of electromagnetic radiation but rather a characteristic of vision and the visual perception of an observer.
• Colour is not an inherent property of electromagnetic radiation but rather a characteristic of vision and the visual perceptions of an observer.
• Colour is what human beings perceive when light is present.
• Objects appear to have different colours to an observer based on the wavelengths and intensity of light when it reaches the retina at the back of the eye.
##### Vision
• When light enters the eye, it interacts with specialized cells called cones in the retina. Cones are responsible for detecting and processing different wavelengths of light, which contribute to our perception of colour.
• The three types of cones, commonly referred to as red, green, and blue cones, respond to different ranges of wavelengths. The combined activity of these cones allows us to perceive a wide range of colours.
• The brain plays a crucial role in the perception of colour. It processes the signals received from the cones and interprets them to create our conscious experience of colour.
• Colour perception is influenced by various factors, including the intensity and quality of light, the surrounding environment, and individual differences in vision.
• Our ability to perceive and differentiate colours provides important cues about the world around us, helping us recognize objects, navigate our environment, and experience the richness of visual stimuli.

There is a difference in meaning between the terms light, electromagnetic radiation, radiant energy and electromagnetic energy in physics.

##### Light
• Light is best used to refer to the subset of electromagnetic radiation that is visible to the human eye, ranging from violet (shorter wavelengths) to red (longer wavelengths).
• Electromagnetic radiation refers to the transfer of all forms of electromagnetic radiation through space by electromagnetic waves and includes gamma rays, ultraviolet (UV), infrared (IR), X-rays, and radio waves, as well as visible light.
• Radiant energy is most commonly used to refer to electromagnetic radiation carried by electromagnetic waves. Radiant energy can be measured using instruments such as photometers, which detect the intensity of light or other forms of electromagnetic radiation.
##### Electromagnetic energy
• Electromagnetic energy is a more general term that refers to any form of energy that is carried by electromagnetic waves, including both radiant energy and other types of energy that are not radiant (e.g., static electric fields).
• The type of energy associated with electromagnetic radiation is a measurable quantity in physics, and its measurement is essential for understanding and analyzing physical systems and processes.
• The unit of measurement for electromagnetic energy in the International System of Units (SI) is the joule (J), which is defined as the amount of energy required to perform one joule of work
• The electronvolt (eV) is another unit of energy commonly used in atomic and subatomic physics.

## Lines normal to one another

###### About lines that are normal to one another
• If one line is normal to another, then it is at right angles.
• In geometry, a normal (or the normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
• In the field of optics, the normal is a line drawn on a ray-tracing diagram perpendicular to (at 900 to), the boundary between two media.
• If the boundary between two media is curved then the normal is drawn at a tangent to the boundary.