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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Solution

The power ( $P$ ) of a lens is given by $P = \frac{1}{f}$
where $f$ is the focal length of the lens in meters.
Given that, the focal length of a lens = $f$ = $+ 40 c m$ = $0.4 m$ .
Therefore the power of the lens given by, $P = \frac{1}{0.4 m} = + 2.5 D$ .
As the power of the lens is positive, it is a converging (convex) lens.
Similarly given that the focal length of a lens = $- 20 c m$ = $- 0.2 m.$
Hence the power of the lens given by $P = \frac{1}{- 0.2 m} = - 5 D$ .
Since the power of the lens is negative, it is a diverging (concave) lens.

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