• Question: I have been stuck with this for ages. Say you have a room, and there is a piece of card, then you shine a light of red on the card from the left and shine a light of green from the right, the 'shadow' created on the left is red, and the 'shadow' created on the right is green, but the space around it is yellow, how does this work? Why are the 'shadows' the colours they are?

    Asked by pates837 to Tim, Mike on 28 Jun 2012.
    • Photo: Tim Stephens

      Tim Stephens answered on 28 Jun 2012:

      Different colours of light mix together by addition. This means that if you shine red light and green light onto a white surface, you get yellow. This is the cause of the yellow around the card.
      The places where only one colour of light falls will be that colour, so the place that’s in shadow on the left (red) side is being lit by the red light, but not by the green (so it appears red). On the right, it’s the opposite.

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