Lenses and how the eye forms an image
Physics · Key Stage 3 · The School
Two shapes, two effects
A CONVERGING lens is thicker in the middle. It bends parallel rays of light inward, to meet at a point called the FOCAL POINT. A DIVERGING lens is thinner in the middle. It bends parallel rays outward, spreading them apart as if they came from a point behind the lens. The shape decides which one happens.
A converging lens can flip the image
Hold an object further from a converging lens than its focal point, and the lens forms a REAL image: it can be caught on a screen, and it is upside down. This is exactly what happens inside a camera and inside your own eye. Move the object very close, inside the focal point instead, and the lens instead produces a magnified, upright image you cannot catch on a screen — a VIRTUAL image, which is what a magnifying glass gives you.
The eye as a lens system
Light enters the eye through the CORNEA, which does most of the initial bending. It then passes through the LENS, which fine-tunes the focus, and forms a real, upside-down image on the RETINA at the back of the eye. The brain then flips this image the right way up. Focusing on something close makes the lens rounder and thicker; focusing on something far away makes it flatter.
In a normal eye, the image lands exactly on the retina. In SHORT SIGHT, the eyeball is slightly too long, or the lens bends light too strongly, so the image forms in front of the retina — distant objects look blurred. A DIVERGING lens in glasses spreads the light out slightly first, pushing the focus point back onto the retina. In LONG SIGHT, the image would form behind the retina, so a CONVERGING lens is added instead, to bend the light inward a little more before it enters the eye. Each type of glasses corrects the SAME underlying problem: getting the image to land in exactly the right place.