Three Glass Panes
Read the puzzle
On a white backing lie three overlapping glass panes: cyan at top left, magenta at top right and yellow lower in the middle. Where cyan and magenta overlap the light looks blue; where cyan and yellow overlap it looks green; where all three overlap it is black. A ? card covers the patch where only magenta and yellow overlap. Which colour hides under the card: A orange, B red, C purple or D brown?
Which colour hides under the ? card?
B RED
Magenta stops green, yellow stops blue: red is left.
How to solve it
Think of each pane as a filter that blocks one colour of light. White light is red, green and blue together. Cyan glass blocks red, magenta glass blocks green and yellow glass blocks blue.
Check this rule against the patches you can see. Cyan and magenta together block red and green, leaving blue, which matches. Cyan and yellow block red and blue, leaving green, which matches. All three block everything, leaving black, which matches too.
Now apply it to the hidden patch. Magenta stops the green, and yellow stops the blue. The only light that gets through both panes is red, so the answer is B, red.
The usual wrong answer is A, orange. A pink-purple pane on top of a yellow one looks as if it should blend like two crayons into orange. But stacked glass does not add colours, it takes them away: each pane can only remove light, so the overlap is always darker and purer than either pane alone. Quick check: red is the one colour both magenta and yellow let through.
