Coloured Shadows

Make a multitude of coloured shadows by blocking or letting through just three coloured lights.

Explanation

When do red and green make yellow? When you’re using lights!

There are two types of colour mixing—additive colour mixing and subtractive colour mixing. If you’ve mixed pigments or paints together to create a different colour, you have tried subtractive colour mixing. In Coloured Shadows, you also get to try some additive colour mixing!

The three primary colours in additive colour mixing are red, green and blue. These colours consist of light that is the particular colour’s wavelength. When the three colours are mixed together, they add up to make white light.

However, if you only mix two of the colours you will get the complementary colour to the light that was removed.

Red and green make yellow (the complement of blue)
Green and blue make cyan (the complement of red)
Blue and red make magenta (the complement of green)

Two additive colours are complementary if they make white when mixed together.

When you look at the different coloured shadows made by your hand in Coloured Shadows, you’re seeing the effect of blocking out one set of lights, leaving the other two lights to mix and create the blocked light’s complement. If you manage to block out two sets of lights, you will see the remaining light’s colour only.

Coloured lights can produce interesting effects in stage-work, such as lighting an actor using blue and yellow lights. The actor appears to be in white light, but their shadow will be blue where they yellow light is blocked out, or yellow where the blue light is blocked out.

If you vary the intensity of each colour, you can generate all the different colours of the rainbow.

Additive mixing is different from subtractive mixing, where you mix coloured pigments or paints to make other colours. Pigments work by absorbing all colours of light except the colour you want to see. As you mix in other pigments, more and more colours of light are absorbed, subtracting the amount of colours you see.

Extras for Experts

We have three types of cone-shaped receptors in our eyes. These cone cells help us to see colour. One type is sensitive to light of low frequencies (red), one type is sensitive to mid-frequencies (green) and the third set is sensitive to light of high frequencies (blue). If all the cones are equally stimulated, we see white.

People with normal colour vision are called trichromats, because they can see the three colours that combine to make up our colour vision. Those who can only see two of the primary colours are dichromates (red-green colour-blind or blue-yellow colour-blind). Monochromates can only see one or none of the primaries.

As well as our colour-sensitive cones, we also have rod-shaped cells in our retinas. These are sensitive to light intensity. Unlike cone cells these work in low-light conditions. This is why our vision is more monochromatic at night.

Things to Try

If you look at a star in the night sky, you might find that it gets dimmer when you try to focus directly on it. This is because the centre of focus of the eye (the fovea) contains mainly cone cells. These don’t work as well in dim light. Instead, to get the brightest view of the stars, you can try to look a little to the side of the star. This allows your light-sensitive rods, which are found outside the fovea, to view the star.

Questions to Ask

Have you ever used a Paint program on your computer and made your own colour? How do you think this worked? How is mixing colour with light different to mixing colour with paints?

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