Visible Light
Light, Sound and Waves

Working with two prisms

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Division and recombination.

This activity shows white light being split into colours, and then that a prism cannot split one of the colours further. In a second experiment, the colours from the first prism are recombined into white light.

What to Prepare

  • A pair of 60 degree prisms per working group
  • A light source for narrow beam of light
  • A small white screen
  • A narrow slot, a few millimetres wide, possibly cut into cardboard, for restricting the beam further

What Happens During this Activity

Pupils arrange the first prism so as to cast a spectrum onto the white screen. We suggest that you check they have managed this before trying out the next steps. Then replace the screen with the slotted card, cutting out all of the colours apart from one that they have selected. They should use the white screen to check that they have managed to select only one colour. Then replace the screen with the second prism, in an attempt to split this colour up further. They will fail! Use the screen to catch the beam after it has left the second prism in order to check that the colour cannot be further subdivided.

Then go back two steps to where the single prism casts a spectrum onto a screen. Now try to insert the second prism so that the colours are recombined into white light. Pupils can be left with this as a puzzle. If stuck, they can be offered a clue: The second prism should reverse the action of the first, so be flipped. Or you could point out that the prisms together need to behave much like the glass block that the pupils looked at earlier and so should be geometrically similar to such a block.

As a result of this experiment, pupils should become more convinced that white light is made up of various different colours.

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