Question

A concave lens is kept in the contact with a convex lens of focal length 20 CM. the combination works as converging lens of focal length 100 cm. the power of concave lens should be​

Answer 1

$\: \: \huge\fbox\pink{ᴀɴsᴡᴇʀ}$

f₁ = focal length of the concave lens.

f₂ = focal length of the convex lens F = The combined focal length.

Then:

1/F = 1/f₁ + 1/f₂

F = 100cm

f₂ = 20 cm

1/100 = 1/f₁ + 1/20

1/f = 1/100-1/20

1/f₁ = -1/25

f₁ = -25cm

The negative sign indicates that the image formed by this lens is virtual.

Power = 1/f₁

= 1/25= 0.04

= $\: \: \fbox{0.04}$

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