LENSES

     A lens is a portion of a transparent medium bounded by two surfaces both of which are usually spherical or one Surface spherical and the other plane. There are two types of lenses. They are

1.CONVEX LENS

A Convex lens is thick at the middle and thin at the outer region. A parallel beam of Light rays. After refraction through a convex lens converges. Therefore, this lens is called as converging lens. 

2.CONCAVE LENS

A concave lens is thin at the middle and thick at the outer region. A parallel beam of light rays after reflection through a concave lens diverges. Therefore, this lens is called as diverging lens.