To see a sharp image, light entering the eye must be focused onto the retina at the back of the eye. The cornea and lens refract incoming light to make this happen. Refraction occurs when light changes direction as it passes between materials in which its speed is different. Curved transparent surfaces can use this effect to make light rays converge or diverge. In short-sightedness, light from distant objects is focused in front of the retina when the eye is relaxed. A suitable diverging lens changes the direction of the incoming rays before they enter the eye, allowing the eye's optical system to focus them farther back onto the retina. In long-sightedness, the eye may have difficulty focusing light from nearby objects. A suitable converging lens can help bring the rays together so that the eye can focus them correctly. The strength of a corrective lens is related to its focal length and is commonly measured in dioptres. An optometrist measures a person's vision to determine the appropriate lens power. Contact lenses use the same physics while sitting directly on the surface of the eye. Similar principles are used in cameras, telescopes and microscopes. In every case, carefully shaped transparent materials control the paths of light rays. Glasses therefore do not simply make objects larger or smaller. They change the direction of incoming light so that the eye can form a sharper image on the retina.