Question

The power of a thin convex lens is +3.5D.It is placed in contact with another thin lens,such that the combined power is +1D.Calculate i)power of second lens
ii) focal length of second lens

Pls explain too.

Answer 1

1) -2.5 because if two lens are in contact then resultant power will be algebric sum of both of them

2) 0.4 because f=1/p

Answer 2

(1) Power of second lens is (-2.5 D).

(2) Focal length of second lens is (-40 cm).

Explanation:

It is given that,

Power of a thin convex lens, $P_1 = +3.5\ D$

Combined power of the combination of lenses, P = +1 D

(1) The combined power of the combination of lenses is given by :

$P=P_1+P_2$

$P_2$ is power of second lens

$P_2=P-P_1$

$P_2=1-3.5$

$P_2=-2.5\ D$

(2) Let $f_2$ is the focal length of second lens

The power of a lens in terms of its focal length is given by :

$P_2=\dfrac{1}{f_2}$

$f_2=\dfrac{1}{P_2}$

$f_2=\dfrac{1}{-2.5}$

$f_2=-0.4\ m$

or

$f_2=-40\ cm$

Hence, this is the required solution.

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