Visible Light and Colour

The continuous range of colours on the electromagnetic spectrum is called the visible light spectrum.

The colour of the Sun is white, and it emits white light. This white light is made up of all the colours on the spectrum. We can use a glass prism to split white light into these different colours.

  • The process of separating white light into its different colours is called dispersion.

The different colours of light in the spectrum are arranged by their wavelengths, from longest wavelength to shortest wavelength.

As the light hits the prism, the different wavelengths of light slow down by different amounts. This means that when they come out the other side, we see the white light as separate colours. Red has the longest wavelength and violet has the shortest.

The order of colours on the spectrum is:

  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet

A rainbow shows the visible light spectrum and you can remember the order of colours in the spectrum using the acronym ROY G. BIV

Seeing Objects

For us to see objects, light must reflect off them and enter our eyes.

How do we see individual colours?

Let’s look at an example with a red surface:

1. White light hits the red surface

2. The red surface absorbs all colours (wavelengths of light) except red

3. Red light reflects into the eye

Mixing Colours

Red, blue and green are the primary colours of light, but you can mix these colours to form secondary colours. This is shown in the diagram below.

  • Mixing red and green forms yellow
  • Red and blue form magenta
  • Green and blue form cyan

When all three primary colours mix, they form white.

How do we see secondary colours?

Let’s consider an example where the surface is of a secondary colour.

1. White light hits a yellow surface

2. The yellow surface absorbs all colours (wavelengths) of light except green and red, which it reflects

3. As the green and red enter our eyes, we see the surface as yellow.

How do we see black and white?

White surface

1. White light hits the white surface

2. The white surface reflects all colours of the spectrum

3. Which means we see the surface as white

Black surface

1. White light hits the black surface

2. The black surface absorbs all the colours of the spectrum

3. So we see the surface as black

Seeing Multiple Colours

Let’s look at when the Sun emits white light onto this plant.

Red light reflects off the petals, so we see the petals as red and green light reflects off the leaves of the plant, making them appear green to us.