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# A Convex Lens of Focal Length 20 Cm is Placed Coaxially in Contact with a Concave Lens of Focal Length 25 Cm. - Physics

ConceptConcave Mirror

#### Question

A convex lens of focal length 20 cm is placed coaxially in contact with a concave lens of focal length 25 cm. Determine the power of the combination. Will the system be converging or diverging in nature?

#### Solution

We have

focal length of convex lens, f1 = +20cm = +0.20 m

focal length of convex lens, f2 = - 25cm = +0.25 m

Equivalent focal length, F = 1/(f_1)+1/(f_1)=1/20+1/(-25)=1/(100)

∴ F = 100cm

Power of convex lens, P_1 = 1/f_1=1/0.20

Power of concave lens, P_2 = 1/f_2 = 1/(-0.25)

Power of the combination, p = p1 + p2

= 1/0.20 +1/-0.25

=100/20 +100/-25

=(500 - 400)/100

=100/100

= 1 D

The focal length of the combination = 1 m = 100 cm

As the focal length is in positive, the system will be converging in nature.

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Solution A Convex Lens of Focal Length 20 Cm is Placed Coaxially in Contact with a Concave Lens of Focal Length 25 Cm. Concept: Concave Mirror.
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