Question

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

the nature and power of each lens​

Answer 1

Explanation:

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

=

f

1

1

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

1

=

0.4 m

1

P_1=2.5\ DP

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

=

f

1

1

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

2

=

0.2 m

1

P_2=-5\ DP

2

=−5 D

Therefore, it is the required solution.

Learn more,

Power of a lens