Question

A convex lens of focal length 20 cm is
placed in contact with a concave lens of
local length 40 cm. The focal length of this
combination of lenses will be:
(a) + 40 cm
(b) + 20 cm
(c) – 40 cm
(d) – 20 cm

Answer 1

1/F = 1/f1 + 1/f2

f1 = +40 cm

f2 = -20 cm

Now do your own math for F

The combination will act like a converging lens if the result is positive

Answer 2

Answer:

Option A

Explanation:

The equation to uses to calculate the focal length of the combination of lenses is-

1/f = 1/f1 + 1/f2

1/f = 1/20 + 1/-40    { focal length of concave lens is always negative}

1/f = 1/20 - 1/40

     taking LCM as 40,

1/f = 2-1/40

1/f = 1/40

so, f = 40 cm

therefore, the combined focal length will be + 40 cm.

Hope it helps!

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