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Is Red Always Red?

When we observe, we use one or more of the five senses to gather information about our surroundings.

Sometimes, our senses can deceive us, our sense of sight is one of these deceivers. Our ability to perceive colour is affected by how long we are exposed to it.

Cones in the human eye pair the following colours:

  • red-green
  • yellow-blue
  • white-black.

This pairing means that if you stare at a red color for 30 seconds or more, the cells in your retina that respond to red will fatigue more and fire less. When you switch over to a white surface, your eyes subtract the red and you see its complementary color green.

The "fatigue" colours appearing without being present is found with the other colour pairing as well- our sense of sight is deceiving!


  • Make observations about how we perceive colour.


  • Per Demo or Class:
    A tabloid-sized (11” x 17”) paper with a drawing of a heart-shaped figure (or other shape) that has a yellow border, a green interior and a small, black dot in the centre
    A tabloid-sized (11” x 17”) white paper with a small black dot in the centre

  • Per Student:
    White paper
    Coloured markers
    Coloured construction paper
    Black paper
    Fluorescent-coloured, adhesive tabs

Key Questions

  • What colour is the interior of the heart?
  • What colour is the border of the heart?
  • What did you see when you looked at the plain white paper?
  • What were the colours compared to the first heart you viewed? Why do you think there was a difference between the two?
  • Relate this activity to the fact that the cones in the human eye pair colours. What colours do you think the human brain pairs together?

What To Do

  1. One by one, have students stare at the coloured heart for 20 seconds without blinking their eyes. Have them focus on the black dot.
  2. Immediately after, have them look at the plain white paper, focussing on its black dot.
  3. Ask students to describe what they see.
  4. Have students create images using white paper, markers, construction paper, glue, black paper and fluorescent coloured adhesive tabs.
  5. Have students look at their images for about 30 seconds and then quickly look away to a blank piece of paper.  What do they see?
  6. Make another image and repeat step 5. Ask students to guess what colours they think they will see.


  • Imagine the colours of an after image. What colours would you use to create the image?

Other Resources

Science World Resources | Est-ce que le rouge est toujours rouge?version français de cette ressource