Question

A concave lens of focal length of 25cm and convex lens of focal length 20cm are placed in contact with each other. find the power of this combination and also calculate focal length of the combination?..........plz ans me as fast as possible.....

Answer 1

Answer:

Hello friend

Explanation:

focal length of concave lens = -25 cm  = -25 /100 m power = P₁ = 1/-25/ 100 = -4 D

focal length of convex lens = +20 cm = 20/100 m power = P₂ = 1/20/100 = 5 D

So net power = P = P₁ + P₂ = -4 D + 5D = 1 D (implies combination behave as convex lens).

Hope it will help you !!

Answer 2

The power of the given combination of lenses is 1 D and the focal length is 100 cm.

Given,

The focal length of the concave lens, $f_1$ = 25 cm

The focal length of the convex lens, $f_2$ = 20 cm

To find,

The power of the given combination of lenses,

and the focal, ($f_c$) length of the same.

Solution,

since

$f_1$ = 25 cm = -25/100 m     (∵ it's a concave lens)

and $f_2$ = 20 cm = 20/100 m

and we know that D (power) = 1/f, so

$D_1$ = 1/$f_1$ = -4 D

$D_2$ = 1/$f_2$ = 5 D

now, the total power of the lenses combined

= $D_1$ + $D_2$

= 1 D

and the focal length is reciprocal of power so,

$f_c$ = 1/($D_1$ + $D_2$ )               (where $f_c$ is the combined focal length)

   = 1/1 m = 100 cm

∴ The power of the given combination of lenses is 1 dioptre and the focal length is 100 cm.