Science Puzzle
Why Can’t You See Colours in the Dark?
Walk round a garden on a moonlit night. You can see the flowers well enough to find your way, but every one of them looks grey. A bright red rose looks almost black. In the morning, all the colours are back.
Why can't you see colours in the dark?
The Answer
Your eyes see colour with cells called cones, and cones need bright light. In dim light only your rods are working, and rods can't tell one colour from another.
Each of your eyes has about 120 million rods and about 6 million cones. Cones come in three kinds, most sensitive to red, green or blue light, and your brain works out colours by comparing how strongly each kind responds. Rods come in only one kind, so they can only report how bright something is. In exchange, they're far more sensitive: rods keep working in light much too dim for cones.
Moonlight isn't colourless. It's sunlight bounced off the Moon, and a camera left open for a long exposure at night shows the garden in full colour. Your eyes just can't collect enough light to see it.
Rods are least sensitive to red, which is why a red rose looks darkest of all at dusk while blue flowers still show up. Rods also aren't found in the very centre of your eye, so astronomers look slightly to one side of a faint star to see it better.
The principle: Rods and cones. Colour vision relies on cone cells, which need bright light; in dim light only the rod cells work, and they can't tell colours apart.