Question

A student uses a lens of focal length 40 cm and another of -20 cm. Write the nature and power of each lens



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Answer 1

First one is convex and 2nd one is concave

1st on has a power of 5/2 D

2nd one has a power of -5 D

Answer 2

The power (P) of a lens is given by P=1f

where fff is the focal length of the lens in meters.

Given that, the focal length of a lens = fff = +40cm+40 cm+40cm = 0.4m0.4 m0.4m.

Therefore the power of the lens given by, P=10.4m=+2.5D.

As the power of the lens is positive, it is a converging (convex) lens.

Similarly given that the focal length of a lens = −20cm-20 cm−20cm = −0.2m-0.2 m−0.2m. 

Hence the power of the lens given by P=1−0.2m=−5D.

Since the power of the lens is negative, it is a diverging (concave) lens.