### KS3 Physics

Motion and Forces
Waves
Matter

# Colour

The colour of the Sun is white, and the Sun emits white light. However, white light is made up of different colours of light. The range of different colours is called a spectrum.

White light can be split into its different component colours using a glass prism.

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

The different colours of light in the spectrum are arranged by decreasing 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, with red having the longest wavelength and violet the shortest wavelength.

The order of colours in 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 using the acronym ROY G. BIV.

## Seeing Objects

For us to see objects, light reflects off them and enters 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 form 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 look at an example when a surface is 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 enters our eye, we will see the surface as yellow.

### How do we see in black and white?

#### For a white surface

1. A 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.

#### For a 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, which is why we see the petals as red. Also, green light reflects off the plant’s leaves, which is why we see the leaves as green.

Quizzes