Convex and concave lenses important in optics because a lens is device that transmit or reflect light. The surface of the lens can be convex, in which case it bulges outwards, or concave, which means that it curves inwards.
Convex lenses are thicker at the middle. Rays of light that passes through the lens are bought closer together. They are said to converge, and so a convex lens is also called a converging lens.
Concave lenses are thinner at the middle. Rays of light that passes through the lens are spread out, and are set to diverge. So , a concave lens is also a called a diverging lens.
Lenses are important in variety of optical instruments, ranging from microscope and telescopes, to spectacles for young and old.