Question

What is the power of a convex lens of focal length 17. What is the power of a concave lens of focal length 25 cm ?

40 cm?​

Answer 1

Step-by-step explanation:

1 ) Power = 1 / focal length

P = 1 / 17 diptometre

Answer 2

Answer:

• The power of a convex lens of focal length 17 is   $5.2283\ diopter$

• The power of a concave lens of focal length 25 cm is $-4\ diopter$

• The power of a concave lens of focal length 40 cm is  $-0.025\ diopter$

Step-by-step explanation:

• Let P be the power and f be the focal length of a lens then,

        $P = \frac{1}{f}$   ...(1)

• That is,

         The power of a lens is given by the reciprocal of its focal length.

• For converging lenses, the focal length will be positive and for diverging lenses, the focal length will be negative

• So for the concave lens,  the focal length is considered to be negative and for the convex lens, the focal length is positive.

In the question, it is given that,

1.    $f = 17\ cm =0.17\ m$

           Using equation (1),

           $P = \frac{1}{0.17} = 5.8823\ diopter$

           so we get the power of the given lens = $5.2283\ diopter$

    2.    $f = -25\ cm =-0.25\ m$

           Using equation (1),

            $P = \frac{1}{-0.25} = -4\ diopter$

           so we get the power of the given lens = $-4\ diopter$

   3.     $f = -40\ cm =-0.40\ m$

           Using equation (1),

            $P = \frac{1}{-0.40} = -0.025\ diopter$

           so we get the power of the given lens = $-0.025\ diopter$