Quiz, myth or fact, and the demos
Test yourself. Every answer comes with its source.
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The colour illusions quiz
1 In which year did the dress photo go viral?
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2015. In February 2015 one photo split the internet: was the dress blue with black lace, or white with gold lace? WIRED
2 Under good white light, what colours is the dress, says Michael Bach?
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Black and blue. Bach’s answer to the real colours, well lit with white light: black and blue. Michael Bach
3 How many red pixels are in Kitaoka’s strawberry tart picture?
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None. There is not a single red, or even reddish, pixel in it. Michael Bach
4 What was Chevreul’s job when he began studying colour contrast?
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Director of dyeing at the Gobelins tapestry works. In 1824 the chemist Michel-Eugène Chevreul became director of dyeing at the Gobelins tapestry works, and soon got complaints that the colours looked dull. Britannica
5 In which year did Chevreul publish The Laws of Contrast of Colour?
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1839. He set it out in 1839 in his most influential book, The Laws of Contrast of Colour. Britannica
6 In Adelson’s checker shadow, how do the two marked squares really compare?
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They have identical luminance. In fact the two squares have identical luminance: the same amount of light comes from each. Michael Bach
7 Who discovered the watercolour illusion, in 1987?
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Baingio Pinna. Baingio Pinna discovered it in 1987. Michael Bach
8 In the lilac chaser, what colour is the spot you see running round?
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Green. Keep your eyes on the cross and you see a green spot running round the ring. Michael Bach
9 According to Bach, what is the opposite colour of red?
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Blue-green. Bach notes that the opposite colour of red is not green, as many textbooks say, but blue-green. Michael Bach
10 About how often do we normally move our eyes?
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About three times a second. We normally move our eyes about three times a second, so afterimages rarely build up in everyday life. Michael Bach
11 What did Charles Benham call his disc in 1894?
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The Artificial Spectrum Top. In 1894 the toy maker Charles E. Benham sold his disc as an ‘Artificial Spectrum Top’. Michael Bach
12 How long can the McCollough effect last after 10 minutes of adapting?
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Months. Ten minutes of adapting is said to give an effect lasting months. Michael Bach
Myth or fact?
Eight statements. Some are true, some are myths.
1 The dress photo has something to do with colour blindness.
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Myth. It has nothing to do with colour blindness. Michael Bach
2 Your brain tries to subtract the colour of the light when it judges colours.
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Fact. Your visual system tries to work out the colour of the light and ‘subtract’ it. This is called colour constancy, and it usually works well. Michael Bach
3 You need red pixels to see something as red.
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Myth. There is not a single red, or even reddish, pixel in it. Michael Bach Swap the strawberries for ovals of the same colours and the ovals still look reddish, so it isn’t only that we know strawberries are red. Michael Bach
4 Two squares can send the same light to your eye and still look different.
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Fact. In fact the two squares have identical luminance: the same amount of light comes from each. Michael Bach Our eyes are not light meters: they take context into account. Michael Bach
5 Colours always look the same, whatever is next to them.
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Myth. His law of simultaneous contrast: colours placed side by side affect each other, each pushing its own complementary colour onto its neighbour. Britannica
6 A photo of Benham’s top spinning shows the colours.
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Myth. Take a photo of it spinning and the colours are not there. Michael Bach
7 An afterimage can appear in the complementary colours.
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Fact. It can be positive, matching the original, or negative: less bright, or in the complementary colours. Britannica
8 Scientists fully understand why Benham’s top shows colours.
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Myth. Nobody knows for certain why the colours appear. Michael Bach