Cones detect colours.
There are around 7 million cones in the human eye.
There are three types of cones, each type detects a different colour, red, green or blue.
When all three types of cones are stimulated equally, we see white.
Rods detect shapes.
Rods help us see at night, when there isn't enough light for the cones to work.
There are around 120 million rods in the human eye.
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You don't have to know what rods and cones look like for the exam, but it might be helpful to see what they look like.
Look at the FOVEA next!