Question

# A double convex lens of focal length 40 cm is bisected vertically. Is there any change in the focal length and power of the lens?

Solution

## The focal length of a lens depend on the extend of convergence it causes.. Incase of a convex lens it has two refractive surfaces…the surface through which ray enter and surface through which it leaves… But when it is cut into half it becomes plano concave lens and hence the radius of curvature of the planer part is infinity… Hence convergence doesnt happen at the planar part..and hence effective focal lenth increases .. In other words a convex lens could be considered as two plano convex lens of focal length f kept together so using formula.. 1/f=1/f1 + 1/f2…we get effective focal length f/2.. hence here the effective focal length = 40/2 = 20 cm. Power become 2pPhysics

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