Like a camera, rays of light enter our eyes through the cornea, passing through the pupil, which is the centre of the iris; then through the lens and finally reaching the retina, that is the film at the back of our eyes. The retina then converts the images into impulses that travel through the optic nerve to the brain.

Clear vision is evident when the cornea and the lens bend the rays of light correctly, focusing them onto the retina. Problematic vision could be due to refractive error, which is a failure to focus light properly by the cornea and natural lens of our eyes. Modern aids such as spectacles, contact lenses and refractive surgery are designed to improve refractive errors by correcting the rays of light focus onto the retina.