Behind the coloured iris of the eye and in line with the pupil is a lens which helps the eye focus light to process an image. In much of the population, this lens is transparent. However, aging, disease, trauma, and environmental factors such as UV light can cause this lens to develop opacities. These changes are called cataracts and they affect the ability of the eye to focus light, much like a fingerprint on glasses or dirt on a window, along with symptoms of glare, halos and colour changes. Once the vision loss is affecting daily activities, cataract surgery may be considered, which has become a highly successful surgical procedure.