Question

What is meant by power of a lens? You have three lenses L1, L2 and L3 of powers +10D, +5D and -10D respectively. State the nature and focal length of each lens. Explain which of the three lenses will form a virtual and magnified image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer

Answer 1

The ability of eye lens to adjust its focal length is called power of accommodation

L1 power= +10D

Power is positive therefore it is a convex lens

f= 1/p = 1/10 =0.1m = 10cm

L2 power = +5D

Power is positive therefore it is a convex lens

f = 1/p = 1/5 = 0.2m = 20cm

L3 power = -10D

Power is negative therefore it is a concave lens

f = 1/p = 1/-10 = -0.1m = -10cm