## Spectrum

Definition

When discussing light,  a spectrum is a linear scale of spectral colours organised by wavelength.

Explanation

A spectrum or spectra (plural) are produced naturally when light is refracted by a prism or by rain.

A diagram of spectral colours is usually presented in the form of a spectrum, a linear band organised by wavelength with red at one end and violet at the other.

Spectra are often shown in an elongated form so that as many different gradations of colour can be seen as possible.

## Speed of light

The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time.

• The speed of light is measured in metres per second (m/s).
• Light travels through a vacuum at 300,000 kilometres per second.
• The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
• Light travels through other media at lower speeds.
• A vacuum is a region of space that contains no matter.
• Matter is anything that has mass and occupies space by having volume.
• When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
• In other contexts empty space is not considered to be a medium because it does not contain matter.

## Speed of light

The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time.

• The speed of light is measured in metres per second (m/s).
• Light travels through a vacuum at 300,000 kilometres per second.
• The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
• Light travels through other media at lower speeds.
• A vacuum is a region of space that contains no matter.
• Matter is anything that has mass and occupies space by having volume.
• When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
• In other contexts empty space is not considered to be a medium because it does not contain matter.

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

## Substance

A substance is a type of matter that has uniform properties.

• A chemical substance is a form of matter that consists of molecules of the same composition and structure.
• A substance cannot be separated into its chemical components without breaking the chemical bonds that hold them together.

## Subtractive colour

A subtractive colour model helps to make sense of what happens when different coloured pigments (eg. paints, inks, dyes or powders) are mixed together to produce other colours.

• CMYK is a subtractive colour model.
• As cyan, magenta, yellow and black (K) pigments are combined they subtract from the amount of colour reflected off the resulting mixture.
• CMYK printing typically uses white paper which has good reflective properties and then adds cyan, magenta, yellow and black ink or toner to produce colour.
• Highlights are produced by reducing the amount of coloured ink and printing without black to allow the maximum amount of light to reflect off the paper through the ink.
• Mid tones rely on the brilliance and transparency of the pigments and the reflectivity of the paper to produce fully saturated colours.
• Shadows are produced by adding black to both saturated or desaturated hues.

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

## Subtractive colour

A subtractive colour model combines different hues of a colourant such as a pigment, paint, ink, dye or powder to produce other colours.

• CMYK is a subtractive colour model.
• CMYK pigments are the standard for colour printing because they have a larger gamut than RGB pigments.
• CMYK printing typically uses white paper which has good reflective properties and then adds cyan, magenta, yellow and black ink or toner to produce colour.
• Highlights are produced by reducing the amount of coloured ink and printing without black to allow the maximum amount of light to reflect off the paper through the ink.
• Mid tones rely on the brilliance and transparency of the pigments and the reflectivity of the paper to produce fully saturated colours.
• Shadows are produced by adding black to both saturated or desaturated hues.

## Sun

The Sun is the star at the centre of our solar system.

Here are some basic facts about the Sun:

• Age: 4.6 Billion Years.
• Star type: Yellow Dwarf (G2V).
• Diameter: 1,392,684 km (109 x Earth).
• Mass: 333,060 x Earth.
• Surface temperature: 5500 °C.
• Internal temperature: 15 million °C.
• Composition: Hydrogen (72%), and helium (26%).
• Energy generation: Thermo-nuclear fusion using hydrogen as fuel.
• Energy production: Equal to 100 billion tons of dynamite per second.
• Electromagnetic radiation emitted by Sun = Solar energy or solar radiation.
• Visible solar radiation = Sunlight.
• Sunlight: Takes 8 minutes to reach Earth (150 million km).
• Wavelengths between red and violet are visible to the human eye.

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

## Sunlight

Sunlight is light emitted by the Sun and is also called daylight or visible light.

• Sunlight is only one form of electromagnetic radiation emitted by the Sun.
• Sunlight is only a very small part of the electromagnetic spectrum.
• Sunlight is the form of electromagnetic radiation that our eyes are sensitive to.
• Other types of electromagnetic radiation that we are sensitive to, but cannot see, are infrared radiation that we feel as heat and ultraviolet radiation that causes sunburn.

## Sunlight

Sunlight is light from the Sun and is also called daylight or visible light.

• Sunlight is only one form of electromagnetic radiation emitted by the Sun.
• Sunlight is the form of electromagnetic radiation that our eyes are sensitive to.
• Other types of electromagnetic radiation that we are sensitive to, but cannot see, are infrared radiation that we feel as heat and ultraviolet radiation that causes sunburn.
• The electromagnetic spectrum includes all possible wavelengths of electromagnetic radiation, ranging from low energy radio waves through visible light up to high energy gamma rays.
• Sunlight is only a very small part of the electromagnetic spectrum.
• The visible part of the electromagnetic spectrum is called the visible spectrum.
• The human eye is tuned to the visible spectrum and so to spectral colours between red and violet.
• 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.

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

## Tangent

A tangent to a circle is a straight line that touches but does not intersect the circle and is at right angles to a radial line drawn from  the centre of the circle.

• In geometry, a tangent (or tangent line) to a curve is a straight line that touches but does not intersect the curve. It can be defined as a line through a pair of infinitely close points on a curve.

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

## Tangent

A tangent to a circle is a straight line that touches but does not intersect the circle and is at right angles to a radial line drawn from  the centre of the circle.

• In geometry, a tangent (or tangent line) to a curve is a straight line that touches but does not intersect the curve. It can be defined as a line through a pair of infinitely close points on a curve.

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

## Temperature

All objects emit electromagnetic radiation, and the amount of radiation emitted at each wavelength depends on the temperature of the object. Hot objects emit more of their light at short wavelengths, and cold objects emit more of their light at long wavelengths. The temperature of an object is related to the wavelength at which the object gives out the most light.

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

## Tone

Tone refers to the perception of a colour’s brightness (intensity).

Tone can also be used to refer to shades of grey between black and white.

• A hue reduced in intensity is called a tone.
• In the context of the HSB colour model, a darker tone of a hue is produced by reducing its brightness (intensity).
• Tone is associated with the term value.
• Value is the amount of light that is reflected from a surface or emitted by a computer screen.
• As the amount of light associated with a hue increases the colour becomes brighter – a lighter tint of that hue.
• As the amount of light associated with a hue decreases the colour becomes a darker –  a darker shade (tone) of that hue.
• In terms of tone, white has a high value, black has a low value.
• The value of an object is usually evaluated relative to the brightness of a similarly illuminated white in the same situation.

## Total internal reflection

Total internal reflection occurs when incoming light traveling through a medium strikes the boundary with a second medium at angles greater than a certain critical angle with the result that no light crosses the boundary and so all the light is reflected back into the medium.

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

## Transverse wave

A transverse wave oscillates (vibrates) from side to side at a right angle to the direction of propagation.

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

## Tristimulus values

The tristimulus values associated with a trichromatic colour space determine the amounts of each of the three primary colours are to be used to select a particular colour.

• The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in short (“S”, 420 nm – 440 nm), middle (“M”, 530 nm – 540 nm), and long (“L”, 560 nm – 580 nm) wavelengths.
• These cone cells underlie human colour perception in conditions of medium and high brightness; in very dim light colour vision diminishes, and the low-brightness, monochromatic “night vision” receptors, denominated “rod cells”, become effective.
• Thus, three parameters corresponding to levels of stimulus of the three kinds of cone cells, in principle describe any human colour sensation. Weighting a total light power spectrum by the individual spectral sensitivities of the three kinds of cone cells renders three effective values of stimulus; these three values compose a tristimulus specification of the objective colour of the light spectrum.
• The three parameters denoted “S”, “M”, and “L”, are indicated using a 3-dimensional space denominated the “LMS colour space”, which is one of many colour spaces devised to quantify human colour vision.
• Stimuli that account for colour perception: can be specified by a set of tristimulus values, defined as the “amounts of the 3 reference colour stimuli, in a given trichromatic system, required to match the colour of the stimulus considered”.

https://en.wikipedia.org/wiki/CIE_1931_color_space#Tristimulus_values