The retina of the eye has two types of light-sensitive cells called rods and cones. Both are found in the retina which is the layer at the back of your eye which processes images. Rods work in low light conditions to help night vision, but cones work in daylight and are responsible for colour discrimination.
There are three types of cone cells and each type has a different sensitivity to light wavelengths. One type of cone perceives blue light, another perceives green and the third perceives red. When you look at an object, light enters your eye and stimulates the cone cells. Your brain then interprets the signals from the cones cells so that you can see the colour of the object. The red, green and blue cones all work together allowing you to see the whole spectrum of colours. For example, when the red and blue cones are simulated in a certain way you will see the colour purple.
The exact physical causes of colour blindness are still being researched but it is believed that colour blindness is usually caused by faulty cones but sometimes by a fault in the pathway from the cone to the brain.
People with normal colour vision have all three types of cone/pathway working correctly but colour blindness occurs when one or more of the cone types are faulty. For example, if the red cone is faulty you won’t be able to see colours containing red clearly. Most people with colour blindness can’t distinguish certain shades of red and green.
