How do your eyes work?
When you look at an object, the light from it enters your eye through the pupil. The iris changes the size of the pupil, depending on how bright the light is. The lens focuses the light onto the back of the eye: the retina. The retina is a mass of light-sensitive neurons, called photoreceptors, which change light signals into electrical ones.
How do photoreceptors work?
What is colour blindness?
The most common form of 'colour blindness' is red-green colour deficiency, which affects 7-10% of people. This is not actually blindness, but a difficulty in distinguishing shades between red and green. It is caused by a change to or loss of the light responsive chemical in certain photoreceptors. Total absence of colour vision, where everything is seen in shades of black and grey, is very rare, and is usually caused by brain damage.