REFRACTION THROUGH A LENS Lens: Lens is a transparent refracting medium bounded by two curved surface which are generally spherical. Types of lens: Generally there are two types of lens: 1. Convex lens: It is a lens which is thicker at the middle and thinner at the edges. It is also called converging lens as the light rays converges on passing through it at a point. Focal length of convex lens is taken as positive. 2. Concave lens: it is a lens which is thicker at the edges and thinner at the middle. It is also known as diverging lens as the ray of light on passing through it diverges at a point. The focal length of concave lens is taken as negative. Bi-convex/Bi-concave lens: The lens with both the surface convex or concave are termed as bi-convex and bi-concave lens respectively. A bi-convex or bi-concave lens has same radius of curvature for both the surfaces but the double convex or double concave lens may or may not have the same radius of curvature for both the surfaces. Action of lens as a set of prism: Convex as well as Concave lens is said to be made by the combination of prism and a glass slab. Convex lens in its upper part has a prism with its base downward and a prism in its lower part has its base upward. When the ray passes through this lens the upper prism bends the ray of light downward and the lower prism bends the ray of light upward that is towards the base of the prism. The central part which is a glass slab passes the incident ray undeviated. Thus the entire ray converges at a point.

Terms related to a lens: 1. Centre of curvature C1 and C2: The centre of the sphere whose part is the lens surface is called centre of curvature. There are two centre of curvature. 2. Radius of curvature R1 and R2: It is defined as the radius of that sphere of which lens is a part. There are two radius of curvature. 3. Principal axis: It is a line joining the two centre of curvatures. 4. Optical centre: It is a point lying on the principal axis of the lens...

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