Observer

A human observer is a person who engages in observation or watches something.

  • In the presence of light, an observer perceives colour.
  • Things appear coloured to an observer because colour corresponds with a property of light that is visible to the human eye.
  • The visual experience of colour is associated with words such as red, blue, yellow, etc.
  • The perception of colour is a very subjective experience.
  • One factor that determines the particular colour an observer sees is the colour of nearby objects.
  • Another factor is to do with the well-being of an observer. Health, medications, mood, emotions or fatigue can all affect the eye, vision and perception.
  • A further factor is the environment in which colours are observed, the type of object and colour associations.
  • Two different observers may see colour differently because of life experience including educational, social and cultural factors.
  • The term observer has distinct and different meanings within the fields of special relativity, general relativity, quantum mechanics, thermodynamics and information theory.

Additive colour

Additive colour is a method of mixing different wavelengths of light to produce other colours.

  • An additive approach to colour is used in the case of emission of light from the screens of mobile phones, computers and televisions.
  • An additive approach to colour is used in the case of the reflection of light off-white, neutral or black surfaces by digital projectors.
  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce all other colours.
  • RGB colour uses three light sources or beams. Each is called a component of the resulting colour,
  • Different colours are produced by varying the intensity of the component colours between fully off and fully on.
  • When fully saturated red, green and blue primary colours are combined they produce white.
  • When any two fully saturated additive primaries are combined they produce a secondary colour: yellow, cyan and magenta.
  • Some RGB colour models can produce over 16 million colours by varying the proportion and intensity of each of the three primary colours.

Angle of reflection

The angle of reflection measures the angle at which reflected light bounces off a surface.

  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Angle of refraction

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.
  • Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction when referring to light passing across the boundary between two different transparent media, such as water, glass, or air.
  • In optics, the law is used in ray diagrams to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a medium.

Bands of colour

An observer perceives bands of colour because:

  • The human eye is able to distinguish between some ranges of wavelengths of visible light better than others.
  • Some ranges of wavelengths appear more intense to a human observer than others.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception.
  • It is the human brain that draws lines between different bands of colour when an observer looks at a rainbow for example.
  • A human observer can distinguish between colours corresponding with many thousands of single wavelengths of light in the visible spectrum. These colours are called spectral colours.
  • There is no property belonging to electromagnetic radiation that causes bands of colour to appear to an observer. The fact that we do see distinct bands is often described as an artefact of human colour vision.
  • The visible spectrum is formed of a smooth and continuous range of wavelengths that can be demonstrated to produce a smooth and continuous range of colours.
  • Cone cells in our eyes are particularly sensitive to red, green and blue wavelengths.
  • Our brains process information received from the eye to produce all the colours of the visible spectrum.

Colour

Things appear coloured because colour corresponds with a property of light that is visible to the human eye. The visual experience of colour is associated with words such as red, blue, yellow, etc.

Colour vision

Colour vision is the ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they emit, reflect or transmit. The human eye and brain together translate light into colour.

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a range of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
  • Colours can be measured and quantified in various ways; indeed, a person’s perception of colour is a subjective process whereby the brain responds to the stimuli that are produced when incoming light reacts with several types of cone cells in the eye. In essence, different people see the same illuminated object or light source in different ways.

Colour wheel

A colour wheel is a diagram based on a circle divided into segments. The minimum number of segments is three with a primary colour in each. Segments added between the primaries can then be used to explore the result of mixing adjacent pairs of primary colours together. Additional segments can then be added between all the existing segments to explore the result of mixing further pairs of adjacent colours.

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of rainbow colours.
  • Colour wheels are often used in technologies which reproduce colour in ways that match the light sensitivity of the three different types of cone cells and the rod cells in the human eye.
  • Colour wheels exploring additive colour models and subtractive colour models use different sets of primary colours.
  • An RGB colour wheel, used to explore additive mixing of light, starts with red, green and blue primary colours.
  • The colours produced in between the primary colours in a colour wheel are called secondary colours.
  • The colours produced in between the secondary colours in a colour wheel are called tertiary colours.
  • A CMY colour wheel, used to explore subtractive mixing of pigments and inks (used in digital printing) starts with cyan, magenta and yellow primary colours.
  • An RYB colour wheel used to explore the subtractive mixing of art pigments and paints starts with red, yellow and blue primaries.
  • The colour wheels described above all depend on trichromatic colour vision which involves three receptor types (cone cells) processing colour stimuli.

Cone cell

Cone cells, or cones, are one of three types of photoreceptor cells (neurons) in the retina of the human eye. They are responsible for colour vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light.

  • Cone cells are cone-shaped whilst rod cells are rod-shaped.
  • Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
  • There are believed to be around six million cone cells in the human retina.

Critical angle

The critical angle for light approaching the boundary between two different media is the angle of incidence above which it undergoes total internal reflection. The critical angle is measured with respect to the normal at the boundary between two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • Internal reflection takes place when light travelling through a medium strikes the boundary of another medium with a lower refractive index at an angle greater than the critical angle.
  • For example, internal reflection takes place when light reaches air from glass at an angle greater than the critical angle, but not when light reaches glass from air.
  • In general, light will be partially refracted and partially reflected because of irregularities in the surface at the boundary.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.

Dispersion

Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.

  • Dispersion is the result of the relationship between refractive index and wavelength.
  • Every wavelength of light is affected to a different degree by the refractive index of a medium and as a result, changes direction by a different amount eg. when passing from one medium (such as air) to another (such as glass). In the case of white light, the separate wavelengths span out with red at one end and violet at the other.
  • Another familiar example of dispersion is when white light strikes raindrops and a rainbow of colours become visible to an observer.
  • As the light first enters and then exits a droplet it separates into its component wavelengths which the observer perceives as colour.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception by an observer.

Electric and magnetic fields

An electric field is created by a change in voltage (charge). The higher the voltage the stronger the field.

A magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.

  • A change in an electric field induces a change in the magnetic field.
  • A change in a magnetic field induces a change in the electric field.
  • An electromagnetic wave is the result of the interaction of electric and magnetic fields.
  • An electromagnetic wave can be propagated when either the charge of an electric field changes or when the current of a magnetic field changes.
  • Once an electromagnetic wave propagates outward it cannot be deflected by an external electric or magnetic field.

Electric field

An electric field is created by a change in voltage (charge). The higher the voltage the stronger the field.

    • Whilst an electric field is created by a change in voltage (charge), a magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.
    • An electromagnetic wave is the result of the interaction of an electric and magnetic field because an electric field induces a magnetic field and a magnetic field induces an electric field.
    • An electromagnetic wave can be induced when either the charge of an electric field changes or when the current of a magnetic field changes or when they both change together.
    • The waveform, wavelength and frequency of an electromagnetic wave result from the rapid periodic succession of transitions between the electrical and magnetic components and the forward propagation of the wave through space.
    • When electric and magnetic fields come into contact to form electromagnetic waves they oscillate at right angles to one another.
    • The direction of propagation of an electromagnetic wave is at right angles to the electric and magnetic fields.
    • The velocity at which electromagnetic waves propagate in a vacuum is the speed of light which is 300,000 metres per second.
    • Once an electromagnetic wave propagates outward it cannot be deflected by an external electric or magnetic field.
    • The reason an electromagnetic wave does not need a medium to propagate through is that the only thing that is waving/oscillating is the value of the electric and magnetic fields.

Electromagnetic field

An electromagnetic field can be thought of as a single more complete object than its component electric and magnetic field. It propagates through space in the form of bundles of energy called photons which are configured as electromagnetic waves, the force carriers of radiant energy (electromagnetic radiation).

  • An electromagnetic field results from the coupling of an electric and magnetic field.
  • When an electromagnetic field experiences a change in voltage or current its reconfiguration into an electromagnetic wave can be described in terms of wavelength, frequency and energy.
  • An electromagnetic wave can be thought to come into existence when a static electric field experiences a change in voltage or a static magnetic field experiences a change in current producing radiating oscillations of electromagnetic energy that propagate through space.
  • The difference between an electromagnetic field and an electromagnetic wave is that the wave has a non-zero frequency component which is the source of the energy it transports.
  • Electromagnetic radiation is essentially the result of an oscillating electromagnetic field propagating through space.

https://en.wikipedia.org/wiki/Electromagnetic_radiation>

Electromagnetic radiation

Electromagnetic radiation is a type of energy that is commonly known as light. Detached from its source, it is transported by electromagnetic waves (or by their quanta, particles called photons) and propagates through space.

  • Electromagnetic radiation includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
  • Electromagnetic radiation is sometimes called EM radiation or electromagnetic radiant energy (EMR).
  • All forms of electromagnetic radiation can be described in terms of both waves or particles.
  • All forms of electromagnetic radiation travel at 299,792 kilometres per second in a vacuum.

Electromagnetic spectrum

The electromagnetic spectrum includes electromagnetic waves of all possible wavelengths of electromagnetic radiation, ranging from low energy radio waves through visible light to high energy gamma rays.

  • There are no precisely defined boundaries between the bands of electromagnetic radiation in the electromagnetic spectrum.
  • 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.
  • Visible light is only a very small part of the electromagnetic spectrum.

Electromagnetic wave

An electromagnetic wave carries electromagnetic radiation.

  • An electromagnetic wave is formed as electromagnetic radiation propagates from a light source, travels through space and encounters different materials.
  • Electromagnetic waves can be imagined as synchronised oscillations of electric and magnetic fields that propagate at the speed of light in a vacuum.
  • Electromagnetic waves are similar to other types of waves in so far as they can be measured in terms of wavelength, frequency and amplitude.
  • We can feel electromagnetic waves release their energy when sunlight warms our skin.
  • Remember that electromagnetic radiation can be described either as an oscillating wave or as a stream of particles, called photons, which also travel in a wave-like pattern.
  • The notion of waves is often used to describe phenomena such as refraction or reflection whilst the particle analogy is used when dealing with phenomena such as diffraction and interference.

Electronvolt

An electronvolt is a unit of energy commonly used to measure the energy carried by electromagnetic radiation.

  • Electronvolts can be used for measurements at the scale of elementary particles as small as single photons, the quantum of the electromagnetic field.
  • One electronvolt is the amount of energy that a single electron has when it is accelerated by a potential difference of 1 volt.
  • If there is a difference in voltage of 1 volt between two points in an electrical circuit (within a capacitor for example) then the force required (and the energy gained) by a photon accelerating from one point to the other is 1 electronvolt.

Energy

Energy is a property of matter.

  • Everything contains energy including all forms of matter and so all objects.
  • Energy is evident in all forms of movement, interaction, and changes to the forms and properties of matter.
  • At an atomic level, energy is evident in the movement of electrons around the nucleus of atoms and energy is stored in the nucleus of atoms as a result of the forces that bind protons and neutrons together.
  • Energy can be transferred between objects, and converted from one form to another, but cannot be created or destroyed.
  • Everything in the universe uses energy of one form or another all the time.
  • When it comes down to it, even matter is a type of energy.
  • Light has energy but no mass, does not occupy space and has no volume.
  • Energy is often described as either being potential energy or kinetic energy.
  • Energy is measured in joules whilst power is measured in joules per second.

Fast medium

Light travels through different media such as air, glass or water at different speeds.  A fast medium is one through which it passes through more quickly than others.

  • Light travels through a vacuum at 299,792 kilometres per second.
  • Light travels through other media at lower speeds.
  • In some cases, it travels at a speed which is near the speed of light (the speed at which light travels through a vacuum) and in other cases, it travels much more slowly.
  • It is useful to know whether a medium is fast or slow to predict what will happen when light crosses the boundary between one medium and another.
  • so:
  • If light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
  • If light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.
  • In optics, the normal is a line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.

Hertz (Hz)

The hertz (symbol: Hz) is a unit used to measure the frequency of electromagnetic waves.

Hexadecimal number

An hexadecimal number (hex number) has a base (radix) of 16 whilst a decimal system of notation has a base of 10.

  • The familiar decimal system of notation uses nine distinct symbols 0 – 9. It then adds columns to the right to denote 10’s, 100’s etc.
  • A hexadecimal system of notation uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F (or a, b, c, d, e, f) to represent values ten to fifteen. Further columns are added on the right to denote 16’s, 256’s etc.
  • A hexadecimal triplet is a six-digit, three-byte hexadecimal system of notation used in programming and software applications (graphic design, web development, photography) to represent colours. The bytes represent the red, then green and then blue components of a colour.
  • Hexadecimal triplets can be used to represent 256 x 256 x 256 different colours.
  • Each byte represents a number in the range 00 to FF in hexadecimal notation (0 to 255 in decimal notation).
  • The hash symbol (#) is used to indicate hex notation.
    • Red = #FF0000
    • Yellow = #FFFF00
    • Green = #00FF00
    • Cyan = 00FFFF
    • Blue = #0000FF
    • Magenta = #FF00FF
  • The sequence of hexadecimal values between 1 and 16 = 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E and F.
  • The sequence of hexadecimal values between 17 and 32 = 10,11,12,13,14,15,16,17,18,19,1A,1B,1C,1D,1E and 1F.
  • The sequence then continues to increment the two digits up to 256.

Internal reflection

Internal reflection takes place when light travelling through a medium such as water fails to cross the boundary into another transparent medium such as air. The light reflects back off the boundary between the two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • For internal refraction to occur, the refractive index of the second medium must be lower than the refractive index of the first medium. So internal reflection takes place when light reaches air from glass or water (at an angle greater than the critical angle), but not when light reaches glass from air.
  • In most everyday situations light is partially refracted and partially reflected at the boundary between water (or glass) and air because of irregularities in the surface.
  • If the angle at which light strikes the boundary between water (or glass) and air is less than a certain critical angle, then the light will be refracted as it crosses the boundary between the two media.
  • When light strikes the boundary between two media precisely at the critical angle, then light is neither refracted or reflected but is instead transmitted along the boundary between the two media.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.
  • The critical angle is the angle of incidence above which internal reflection occurs. The angle is measured with respect to the normal at the boundary between two media.
  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, 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.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Law of refraction (Snell’s Law)

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).

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 source

A light source is a natural or man-made object that emits one or more of 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.

Magnetic field

A magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.

  • Whilst an electric field is created by a change in voltage (charge), a magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.
  • An electromagnetic wave is the result of the interaction of an electric and magnetic field because an electric field induces a magnetic field and a magnetic field induces an electric field.
  • An electromagnetic wave can be induced when either the charge of an electric field changes or when the current of a magnetic field changes or when they both change together.
  • The waveform, wavelength and frequency of an electromagnetic wave result from the rapid periodic succession of transitions between the electrical and magnetic components and the forward propagation of the wave through space.
  • When electric and magnetic fields come into contact to form electromagnetic waves they oscillate at right angles to one another.
  • The direction of propagation of an electromagnetic wave is at right angles to the electric and magnetic fields.

Medium

Any material through which an electromagnetic wave propagates (travels) is called a medium (plural media).

  • In optics, a medium is a material through which electromagnetic waves propagate.
  • Although electromagnetic radiation is able to propagate through a wide range of media, it is not dependent upon on any medium for propagation and travels at the speed of light through a vacuum.
  • The reason an electromagnetic wave does not need a medium to propagate through is because the only thing that is waving/oscillating is the value of the electric and magnetic fields.
  • In general terms, empty space (a vacuum) is not considered to be a medium because it does not contain matter.
  • It is the permittivity and permeability of a medium that determines how waves travel.

Normal

If one line is normal to another, then it is at right angles. So in geometry, the normal is a line drawn perpendicular to and intersecting another line.

In optics, 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.

  • Light travels in a straight line through a vacuum or a transparent medium such as air, glass, or still water.
  • If light encounters a force, an obstacle or interacts with an object, a variety of optical phenomena may take place including absorption, dispersion, diffraction, polarization, reflection, refraction, scattering or transmission.
  • Optics treats light as a collection of rays that travel in straight lines and calculates the way in which they change direction (deviate) when encountering different optical phenomena.
  • When the normal is drawn on a ray diagram, it provides a reference against which the amount of deviation of the ray can be shown.
  • The normal is always drawn at right angles to a ray of incident light at the point where it arrives at the boundary with a transparent medium.

https://en.wikipedia.org/wiki/Geometrical_optics

Object

An object is a material thing that can be seen and touched.

  • An object is intuitively assumed to exist and to be responsible for a unified experience, consisting of visual and other sensations and perceptions.
  • Every object, material, medium or substance that we can see is made of matter of one kind or another. The key differentiating factor is the elements and molecules they are constructed from.
  • You will have come across the elements that make up the periodic table.
  • A close look at molecules reveals that they are made up of atoms composed of electrons surrounding a nucleus of protons and electrons.
  • Light illuminates objects. In a nutshell, different elements and molecules react to light in different ways because of their atomic structure and the particular way they combine to form mixtures or compounds.
  • In the case of an opaque object, it is the molecules that form its surface that determine what happens when light strikes it. Translucent and transparent objects behave differently because light can travel through them.
  • Another factor that needs to be taken into account when light strikes an object is surface finish. A smooth and polished surface behaves differently from one that is rough, textured or covered in ripples.

Optical phenomena

Optical phenomena result from the behaviour and properties of light, including its interactions with matter. They include absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.

  • Optics is the branch of physics which describes the behaviour of visible, ultraviolet, and infrared light.
  • Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
  • Most optical phenomena can be accounted for using the classical electromagnetic description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice.
  • Practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.
  • Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics.
  • Some phenomena depend on the fact that light has both wave-like and particle-like properties. Explanation of these effects requires quantum mechanics.
  • When considering light’s particle-like properties, the light is modelled as a collection of particles called photons.
  • Quantum optics deals with the application of quantum mechanics to optical systems.
  • Practical applications of ray diagrams are found in relation to a variety of technologies and descriptions of how everyday objects work, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics.

Oscillation

An oscillation is a periodic motion that repeats itself in a regular cycle. An oscillating movement is always around an equilibrium point or mean value. It is also known as a periodic motion.

Photon

Electromagnetic waves are carried by particles called photons.

Photon energy

Photon energy is the energy carried by a single photon. The amount of energy is inversely proportional to the wavelength and directly proportional to the photon’s electromagnetic frequency.

  • The higher the photon’s frequency, the higher it’s energy. Equivalently, the longer the photon’s wavelength, the lower it’s energy.
  • Photon energy is solely a function of the photon’s wavelength and frequency.
  • Other factors, such as the intensity of the radiation, do not affect photon energy. In other words, two photons of light with the same colour and therefore, same frequency, will have the same photon energy, even if one was emitted from a wax candle and the other from the Sun.

Primary colour

Primary colours are a set of colours from which others can be produced by mixing (pigments, dyes etc.) or overlapping (coloured lights).

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to the electromagnetic spectrum that results in the perception of colour.
  • A set of primary colours is a set of pigmented media or coloured lights that can be combined in varying amounts to produce a wide range of colour.
  • This process of combining colours to produce other colours is used in applications intended to cause a human observer to experience a particular range of colours when represented by electronic displays and colour printing.
  • Additive and subtractive models have been developed that predict how wavelengths of visible light, pigments and media interact.
  • RGB colour is a technology used to reproduce colour in ways that match human perception.
  • The primary colours used in colour-spaces such as CIELAB, NCS, Adobe RGB (1998) and sRGB are the result of an extensive investigation of the relationship between visible light and human colour vision.

Rainbow

rainbow is an optical phenomenon produced by illuminated droplets of water. Rainbows are caused by reflectionrefraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

  • Rainbows can be produced by meteorological phenomena, waterfalls, lawn sprinklers and other things that create a fine mist of water.
  • A rainbow is formed from millions of individual raindrops each of which reflects and refracts a tiny image of the sun towards the observer.
  • It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
  • If the sun is behind an observer then the rainbow will appear in front of them.

https://en.wikipedia.org/wiki/Rainbow

Rainbow colours

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

  • The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
  • Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
  • Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
  • Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.

https://en.wikipedia.org/wiki/ROYGBIV

Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.

If the reflecting surface is very smooth, the reflected light is called specular or regular reflection.

Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of objects that the light has already encountered become visible to an observer.

Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in all directions and so no image is produced.

Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

  • As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
  • As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
  • In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
  • A fast (optically rare) medium is one that obstructs light less than a slow medium.
  • A slow (optically dense) medium is one that obstructs light more than a fast medium.
  • The speed at which light travels through a given medium is expressed by its index of refraction.
  • If we want to know in which direction light will bend at the boundary between transparent media we need to know:
  • Which is the faster, less optically dense (rare) medium with a smaller refractive index?
  • Which is the slower, more optically dense medium with the higher refractive index?
  • The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
  • Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.

Refractive index

The refractive index of a medium is the amount by which the speed (and wavelength) of electromagnetic radiation (light) is reduced compared with the speed of light in a vacuum.

  • Refractive index (or, index of refraction) is a measure of how much slower light travels through any given medium than through a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • The refractive index of a medium is a numerical value and is represented by the symbol n.
  • Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
  • If the speed of light in a vacuum = 1. Then the ratio is 1:1.
  • The refractive index of water is 1.333, meaning that light travels 1.333 times slower in water than in a vacuum. The ratio is therefore 1:1.333.
  • As light undergoes refraction its wavelength changes as its speed changes.
  • As light undergoes refraction its frequency remains the same.
  • The energy transported by light is not affected by refraction or the refractive index of a medium.
  • The colour of refracted light perceived by a human observer does not change during refraction because the frequency of light and the amount of energy transported remain the same.

https://en.wikipedia.org/wiki/Refractive_index

RGB colour values

RGB colour values are expressed as decimal triplets or hexadecimal triplets and are used in software applications to select specific colours colour.

  • RGB colour values (codes) are represented by decimal triplets (base 10) or hexadecimal triplets (base 16).
  • In decimal notation, an RGB triplet is used to represent the values of red, then green, then blue. A range of decimal numbers from 0 to 255 can be selected for each value.
    • Red = 255, 00, 00
    • Yellow = 255, 255, 0
    • Green = 00, 255, 00
    • Cyan = 00, 255, 255
    • Blue = 00, 00, 255
    • Magenta = 255, 00, 255
  • In hexadecimal notation an RGB triplet is used to represent the value of red, then green, then blue. A range of  hexadecimal numbers from 00 to FF can be selected for each value.
  • The hash symbol (#) is used to indicate hex notation.
    • Red = #FF0000
    • Yellow = #FFFF00
    • Green = #00FF00
    • Cyan = #00FFFF
    • Blue = #0000FF
    • Magenta = #FF00FF
  • The sequence of hexadecimal values between 1 and 16 = 0,1,2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F.
  • The sequence of hexadecimal values between 17 and 32 = 10,11,12,13,14,15,16,17,18,19,1A,1B,1C,1D,1E and 1F.

RGB colour

  • To be clear about RGB colour it is useful to remember first that:
    • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
    • A spectral colour is a colour corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
    • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. However, because of the way the eye works, we can see many other colours which are produced by mixing colours from different areas of the spectrum. A particularly useful range of colours is produced by mixing red, green and blue light.
    • RGB colour is an entirely different approach to producing and managing colour.
  • RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.
  • Except for the three primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
  • RGB colour provides the basis for a wide range of technologies used to reproduce digital colour.
  • RGB colour provides the basis for reproducing colour in ways that are well aligned with human perception.
  • When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue coloured lights they are able to create a match for a very extensive range of colours.
  • When looking at any modern display device such as a computer screen, mobile phone or projector we are looking at RGB colour.
  • Magenta is an RGB colour for which there is no equivalent spectral colour.

https://en.wikipedia.org/wiki/Comparison_of_color_models_in_computer_graphics

ROYGBV

ROYGBV is an acronym for the sequence of hues (colours) commonly described as making up a rainbow: red, orange, yellow, green, blue, and violet.