• The formula used when a light ray enters a medium with refractive index n2 from a medium with refractive index n1 at curved interface with a radius of curvature R is n₂/v –n₁/u = (n₂- n₁)/R


• A lens is formed when one medium is separated from another medium by two surfaces, one of which is curved.


• Lens formula is 1/f =1/v – 1/u
  where f is the focal length of lens, u is the object distance and v is the image distance.


• Lens maker’s formula is 1/f = (n-1)(1/R₁-1/R₂)
  where R1and R2 are radii of curvature , n is the refractive index and f is the focal length.


• Characteristics of the image formed due to convex lens,
  

Reflections on concepts

1. Write the lens maker’s formula and explain the terms in it.


2. How do you verify experimentally that the focal length of a convex lens is increased when it is kept in water?


3. How do you find the focal length of a lens experimentally?


4. Draw ray diagrams for the following positions and explain the nature and position of image.
   
   i. Object is placed at C2
   
   ii. Object is placed between F2 and optic centre P.