Lenses are pieces of transparent material, such as glass or plastic, which have been made into special shapes. They refract (bend) light in certain ways depending on their shape. Lenses may be convex or concave. Convex lenses are thicker in the middle than they are at the edges. Concave lenses are thinnest in the middle.
A convex lens
Light rays from a small, close object travel in straight lines to the lens. But as they pass through the lens and towards your eye, they bend inward. Since your brain expects light to travel in straight lines, you see a magnified (larger) image.
A concave lens
Rays of light from a tennis ball travel in straight lines to the lens. As they pass through the lens, they bend outward towards your eyes. Again, the brain expects these rays to have arrived in straight lines and you see a smaller image.
Convex and concave lenses are very useful. They are found in many of the instruments which help us to see things which we could not see with our eyes alone. Lenses are used in telescopes which help us see stars and planets, in binoculars which enable us to watch birds and animals in the wild, and in microscopes which magnify tiny living things.
People use lenses to carry out detailed work.
Picture Credit : Google