The working of a lens is based on the refraction of light rays when they pass through it .
A lens is a piece of transparent glass bound by two Spherical surfaces.
There are two type of lenses. Convex lens and concave lens.
(i) convex lens : A convex lens is thick at the centre but thinner at the edges and a piece of transparent Spherical glass having a bulge in the middle. Converting lens
Convex lens are mainly of three types :
(a). Double convex lens : Lenses that has both the surface convex.
(b). Plano-convex Lens : Lenses that has one surface plane and the other surface convex.
(c). Concave-convex Lens : Lenses that has one surface concave and the other surface convex.
(ii) Concave lens : A concave lens is thin in the middle but thicker at the edges and a piece of transparent Spherical glass having a depression in the middle. Diverging lens.
(a) Double Concave Lens : It has both the surfaces concave.
(b) Plano- concave Lens : It has one surface plane and the other surface concave.
(c) Convexo-Concave Lens : It has one surface convex and the other surface concave.
Some Important Terms Related to Spherical Lenses :
(i) Radius of curvature (R) :
The radius of curvature of the surface of a lens is defined as the radius of the sphere of which each surface of lens is a part R1 and R2 in the diagram are radii of curvature of the lenses.
(ii) Centre of curvature (C) :
The centre of curvature of the each surface of a lens is the centre of the spheres of which each surface of the lens is a part . Because a lens has two surfaces , so it has two centre of curvature. In the diagram C1 and C2 are the centre of curvature.
(iii) Principal axis (C1C2) :
It is the line joining the two centres of curvature (C1 and C2) of the lens :
(iv) Aperture : Aperture of a lens is the actual diameter of the circular outline of the spherical lens.
(v) Optical Centre : A point on the axis of a lens that is so located inside the lens that any ray of light passing through it suffers no net deviation.
If the radii of curvature of the two surfaces are equal , then the optical centre coincides with the geometric centre of the lens.
(vi) Principal focus and focal length :
(a) First principal focus and first focal length :
It is a point on the principal axis of the lens such that the rays emerging from this point ( in case of convex lens ) or appearing to converge towards this point ( in case of concave lens ) , after refraction from the lens , become parallel to the principal axis. It is represented by f1.
(b) Second principal focus and second focal length.
It is a point on the principal axis of the lens such that the light rays incident parallel to the principal axis , after refraction from the lens , either converge to this point ( in case of convex lens ) or appear to diverge from this point ( in case of concave lens ) . It is denoted by f2.
In case the medium on both sides of the lens is same , then the first and second focal length are equal magnitude . Thus,
f = f'