Question

A student uses a lens of focal length 40 cm and other of - 2020 m write the nature power of its lens

Answer 1

F=40cm(convex lens)

F=-2020m(concave lens)

Answer 2

Power of the first and second lens is 2.5 D and -5 D respectively.

Explanation:

The focal length of convex lens is positive. The first lens used by the student is convex having focal length of 40 cm or 0.4 m

Power of the first lens is,

$P_1=\dfrac{1}{f_1}$

$P_1=\dfrac{1}{0.4\ m}$

$P_1=2.5\ D$

The focal length of concave lens is negative. The second lens used by the student is concave having focal length of -20 cm or -0.2 m

Power of the first lens is,

$P_2=\dfrac{1}{f_1}$

$P_2=\dfrac{1}{0.2\ m}$

$P_2=-5\ D$

Therefore, it is the required solution.

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