Visible light, colour and scattering

​​In a nutshell

Visible light is the frequency range of the electromagnetic (EM) spectrum that humans can see. Humans see colour because objects reflect or absorb different frequencies of light. 

Definitions

Colour

Visible light is the frequency range of the EM spectrum that humans can see. Within visible light, different colours have different frequencies and wavelengths.

Visible light spectrum

Tip: Use the mnemonic ROYGBIV ("Roy-Gee-Biv") to help remember the order of the colours!

The primary colours are Red, Green and Blue. If an object is a different colour, it will either be made up of a combination of the primary colours' wavelengths, or the specific wavelength of the colour.

Example

Violet will either be a combination of red and blue wavelengths, or its own violet wavelength ($$380\,nm$$​).

The colour of an opaque object corresponds to the wavelengths of light it absorbs and reflects. Objects strongly reflect the colour they are made up of, and absorb any other colours.

Example

A green lime will reflect green wavelengths of light but absorbs any other wavelength.

White objects reflect all wavelengths of light. This is because white light is made up of all wavelengths of light. 

Black objects absorb all wavelengths of light and reflect none. Black is the absence of light. 

Curiosity: Black and white are not considered colours as they don't have specific wavelengths!

Most objects are opaque and not a primary colour. Consider a yellow object. When white light is shone onto the yellow object, the object absorbs the blue light and reflects the green and red light. This makes the object appear yellow. 

This yellow colour is made up of a mixture of green and red wavelengths but the yellow will also have its own unique wavelength too. This wavelength will be somewhere between the wavelength for green light and for red light.

Colour filters

​Objects that aren't opaque transmit light. This means that some (or all) of the light will pass through the object.
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Colour filters allow only certain wavelengths of light to be transmitted. The wavelengths of light it allows through matches the colour of the filter and absorbs all other wavelengths. This means objects viewed through a colour filter may appear differently.

Example

The green lime viewed through a green filter will look the same as it does through no filter.

However, if you viewed it through a red filter, no green wavelengths will transmit through so it will appear black.

Scattering

Light can be scattered by small particles in a liquid or in the air. Scattered means that the light is directed in many directions when it interacts with a particle. 

Example

The small molecules in the Earth's atmosphere scatter blue light the most. When light reaches the Earth from the Sun, blue light is scattered the most so the sky appears blue.