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Coloured Shadows

In this demonstration, students experiment with red, green, and blue light to get different shadow colours.

Primary colors are not a fundamental property of light but are related to the physiological response of the eye to light. Light is a continuous spectrum of all of the wavelengths that can be detected by the human eye.

However, the human eye normally contains only three types of colour ranges of the colour spectrum.

Humans and other species with three such types of color receptors are known as trichromats. The R, G, B cones are most sensitive to the red, green, and blue wavelengths, respectively. Note that these are the colours each receptor is the most sensitive to. The blue cone will also pick up purple, blue, and green hues. Green cones will pick up purple, blue, green, yellow, and orange hues. Red cones will pick up purple, blue, green, yellow, orange, and red hues (see diagram below). The sensitivities of the three cones overlap to let us perceive all visible colours.

When red light, blue light, and green light stimulate the three colour receptors on our retinas equally, the signal gets blended in our brain and we see "white" light. This is why we see sunlight as white light, not as its individual colours.

Slight changes in the relative signals of the cones enable us to see a myriad of different colour combinations, which is why we "see" more than just three colours.

In this demonstration, coloured filters are affixed to flashlights.

The coloured filters block some wavelengths and let others through. The red-filtered flashlight puts out wavelengths from red through to orange and yellow. The green-filtered flashlight puts out yellow, green and blue wavelengths. The blue-filtered flashlight puts out green, blue, and purple wavelengths. When the three coloured light bulbs all shine on the same surface, we see all the wavelengths from red through to purple, which we perceive as white.

With these three coloured lights you can make shadows of seven different colours: blue, red, green, black, cyan (blue-green), magenta (pink), and yellow.

If you block two of the three lights, you get a shadow of the third color.

  • block the red and green lights and you get a blue shadow.
  • block all three lights, you get a black shadow.
  • block one of the three lights, you get a shadow whose colour is a mixture of the two other colours.

Note: If the blue and green mix, they make cyan; red and blue make magenta; red and green make yellow.

Objectives

  • Describe how different colours of light mix to give new colours.

Materials

  • Per Group:
    scissors
    tape
    3 flashlights
    books
    3 coloured cellophane sheets in red, yellow, and blue
    3 pieces of black cardstock
    plasticine
    pencil
    white wall or screen
    dark room

Key Questions

  • How have the flashlights been altered?
  • What colour do you see when all three flashlights shine on the same spot?
  • Do you get the same colours when red, green, and blue paints/inks are mixed? Why not?

What To Do

Set up

  1. Cut out a red, green, and blue circle of cellophane and tape one over each flashlight.
  2. Using the books as supports, arrange the flashlights so that they all shine together on one part of the white wall.
  3. Stand up a pencil (or another object), between the flashlights and the wall and secure with the plasticine. This is what will cast the shadow.

Demonstration

  1. Turn on all 3 flashlights against the white screen/wall.
  2. Block all 3 lights using the pieces of black card.
  3. Block combinations of the three lights, and ask the students to predict the colour of the pencil’s shadow.

Teacher Tip: Instead of supporting the flashlights on books, have volunteers hold the flashlights. Alternatively, just place the coloured cellophane on an overhead projector.

Extensions

  • How do colour filters work?

Other Resources

Olympus Life Sciences |How do colour filters work?