Light frequencies, prisms and colour

In a nutshell

The frequency of a light wave is related to its colour. Each colour has its own frequency and wavelength. White light is made up of all frequencies of visible light. Different colour light refracts at different angles, and white light can be split up by using a prism.

Frequencies of light

Visible light waves have a range of different frequencies. These frequencies correspond to the colour of the wave. Frequency and wavelength are related so each colour also has a certain wavelength of light too.

Example

Wavelength of blue light: $450\,nm$ 

Wavelength of red light: $650\,nm$ 

Curiosity: $1 \,nanometre$  ( $nm$ ) is $1$ billionth of a $metre$ !

There is a spectrum of colours we see as visible light, ranging from red to violet. Visible light colours can be seen in a rainbow.

Science; Waves; KS3 Year 7; Light frequencies, prisms and colour

Red light waves have the longest wavelength but smallest frequency. Violet waves have the smallest wavelength but largest frequency.

A useful mnemonic to remember the rainbow is ROYGBIV ("Roy-Gee-Biv"). It's the same to remember the order of the colours going from lowest frequency to highest.

R	Red
O	Orange
Y	Yellow
G	Green
B	Blue
I	Indigo
V	Violet

There are three primary colours: red, blue and green. All other colours are made up of a combination between these three colours. This is true for colours of light waves as well.

Note: The primary colours are different to the ones used in art (red, blue and yellow)!

When an object is a certain colour, it will absorb all other frequencies of light and reflect its own.

Example

A blue water bottle will reflect blue light waves but absorb the rest. As it absorbs other colours, we can only see the light that is reflected - the colour blue.

If you were to shine a red light on that blue water bottle, it would appear black! That's because there is no blue light for the bottle to reflect, and it would absorb the red light.

White and black light

White light is not on the colour spectrum of light. This is because it contains every frequency of visible light. Black is the absence of any frequency of light.

White objects will reflect every frequency of light. Black objects will absorb all frequencies of light.

Tip: When it's very sunny, wearing all black will feel hotter than all white. The black material absorbs light from the sun and transfers it to thermal energy. So remember to wear white to stay cool!

Prisms and dispersion

Different frequencies of light refract at different angles. Dispersion is the spreading out of frequencies of light caused by refraction of light. Prisms are triangular blocks of glass that are able to achieve dispersion of light.

Science; Waves; KS3 Year 7; Light frequencies, prisms and colour

White light enters the prism. It refracts at the boundary between air and glass. Different frequencies refract at different angles - this is dispersion. It refracts again at the boundary between glass and air and disperses even more.

Example

Rainbows: The light from the sun is refracted at different angles through raindrops.