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Why things have colour

Waves, sound and light · level 6, lesson 3 of 6

A question to start

Mars is called the Red Planet, and you can see its colour with your own eyes in the night sky. What makes Mars red?

The idea

White light contains all the colours. When it falls on a surface, some colours are absorbed and the rest are reflected. The colour you see is the colour that is reflected.

  • A red apple reflects mostly red light and absorbs the rest.
  • A white page reflects all colours about equally.
  • A black coat absorbs almost all of them.

A filter works the same way for light passing through: a red filter lets red through and absorbs the other colours.

Mars is red because iron minerals in its dust and rocks have oxidised, rusted, as NASA puts it. Rust absorbs blue and green light and reflects red and orange.

white light inred reflectedrusty dustblue, green absorbed

Worked examples

1. A green leaf reflects green and absorbs red and blue, which the plant uses for energy.

2. Through a red filter, a blue object looks black: the filter blocks the blue light coming from it.

3. Under red light, a white page looks red, because red is the only colour there to reflect.

Try it

  1. Why does a banana look yellow?
  2. What colour does a white shirt look under green light?
  3. Why does Mars look red?
  4. A red rose is seen through a blue filter. What colour does it look?
Answers
  1. It reflects mostly yellow light and absorbs the other colours.
  2. Green.
  3. Iron minerals in its dust and rocks have rusted; rust reflects red and absorbs blue and green.
  4. Black: the rose reflects red, which the blue filter blocks.

Common mistake

"Objects make their own colour." Most objects only reflect colours that are already in the light falling on them. In the dark they have no colour at all.

Where it leads