than at the edges. A parallel beam of light passing through it, is diverged or spread out. So, a concave lens is also called as diverging lens. .
. Other types of Lenses Plano-convex lens: If one of the faces of a bi-convex lens is plane, it is known as a plano- convex lens. Plano-concave lens: If one of the faces of a bi-concave lens is plane, it is known as a plano-concave lens. All these lenses are shown in Figure .
given below: Figure . Types of lenses . IMAGES FORMED DUE TO REFRACTION THROUGH A CONVEX AND CONCAVE LENS When an object is placed in front of a lens, the light rays from the object fall on the lens. The position, size and nature of the image formed can be understood only if we know certain basic rules.
Rule- : When a ray of light strikes the convex or concave lens obliquely at its optical centre, it continues to follow its path without any deviation (Figure . ). Figure . Rays passing through the optical centre Rule- : When rays parallel to the principal axis strikes a convex or concave lens, the refracted rays are converged to (convex lens) or appear to diverge from (concave lens) the principal focus (Figure .
). Figure . Rays passing parallel to the optic axis Rule- : When a ray passing through (convex lens) or directed towards (concave lens) the principal focus strikes a convex or concave lens, the refracted ray will be parallel to the principal axis (Figure . ).
Figure . Rays passing through or directed towards the principal focus . REFRACTION THROUGH A CONVEX LENS Let us discuss the formation of images by a convex lens when the object is placed at various positions. Object at infinity When an object is placed at infinity, a real image is formed at the principal focus.
The size of the image is much smaller than that of the object (Figure . ). M N O C F F C Figure . Object at infinity Object placed beyond C (>2F) When an object