Question

define power of a lens and write its SI unit a convex lens of power 4D is placed at a distance of 40 cm from a wall at what distance from the lens should a candle be placed so that its image is formed on the wall ​

Answer 1

Explanation:

it is defined as the reciprocal of its focal length.

si unit is dioptre( D)

f = 1/ 4 =0.25 m = 0.25 x100cm = 25 cm

from the above statement.

V = 40cm

1/f = 1/v-1/u

sign convention

1/f = - 1/v - 1/u

1/25 = - 1/40 - 1/u

1/u = - (1/25 +1/40)

1/u = - 65/(25x 40)

u = - (25 x 40)/65

u = - ( 5x 40)/13

u = - 200/13 cm