Questions about the eye
- What determines eye colour?
Your eye colour is determined by the amount of melanin in the cells of the outside layer of your iris. Everybody has roughly the same amount of these cells - melanocytes - but the amount of melanin they contain depends on the individual.
Melanin is a natural substance that absorbs light, giving colour to hair, skin and eyes. The level of melanin we have, and our eye colour, is partly determined by our genes.
If there is a large amount of melanin in your iris cells, you are likely to have brown eyes and darker coloured skin and hair. If there is little, you are more likely to have blue eyes, pale skin and blonde hair.
The actual colour of the eyes for those with less melanin is yellow, but it appears as blue or grey because of light being reflected back by collagen. Collagen inside the iris scatters and absorbs all colours apart from blue or grey, which bounce back through the top layer of the iris.
- What is the blind spot?
The blind spot is the point in the eyeball at the head of the optic nerve. This is where nerve fibres and blood vessels leave the eye, and as a result there are no photosensitive cells. Any images, therefore, that are projected onto the blind spot are simply not registered by the brain.
- Why do pupils dilate?
The pupil is a circular hole in the middle of the iris which regulates the amount of light passing through to the retina. In dark conditions the pupil expands - or 'dilates' - to allow as much light as possible pass through. In bright conditions the pupil shrinks, limiting the amount of light passing through.
dim light bright light
Our pupils also dilate when we rapidly change from 'distant' to 'near' focus, and when we are aroused by things that attract or interest us. Certain drugs may also cause dilation.
- Why do we blink?
Blinking helps to protect our eyes by keeping them wet. When we blink we spread tears from one side of our eye to the other to stop them drying out.