Lenses

Transparent objects with an evenly round surface are called lenses if they redirect light in a certain way. Usually lenses are made of glass or transparent plastic. They have to be transparent because light has to be able to pass through the material. They also must have a different refraction index than the material or the substance which surrounds the lens. The principle on which a lens's function is based is the refraction light. Light rays are refracted each time at the interface between lens and surrounding material; when the light ray enter and when the light rays leave the lens.
Depending on the lens's shape the light rays are refracted such that they either converge or diverge behind the lens. Based on that fact the two basic shapes lenses can have are called: converging lenses or diverging lenses.

Converging Lenses

Lenses that are curved on the outside are called converging lenses. Light rays which pass through converging lenses are refracted in such a way that they move stronger towards one point after they leave the lens. If parallel light rays enter a lens, they cross each other in one point behind the lens - light is focused, the light rays converge. This is depicted in the picture above. Note the area where scorch marks were possible if the screen would be positioned in this area. The result of the lens's effect is a much higher density of light in this area than before.
Every lens has its individual distance towards a point where most of the entering parallel light rays are focused one single point - the focal point. The precision in which a lens is able to focus light on one single point also depends on the shape of the lens. If the shape is too flat or too round, there will be no precise focal point. Then such a lens only has a focal area.

Diverging Lenses

Lenses that are curved on the inside are called diverging lenses. This shape causes a refraction of light rays in such a way that they move apart after they have passed the lens - light is scattered, the light rays diverge. This happens especially strongly if parallel light rays enter the lens.