Question

QUESTION:-
Two thin convex lenses of focal lengths 20cm and 5cm respectively, are placed at a distance d. If a parallel beam incident on the first lens emerges as a parallel beam for second lens, then the value of "d" is
A) 5cm B) 15cm C) 20cm D) 25cm

Answer 1

The value of distance is 25 cm.

Option (D) is correct.

Explanation:

If the lenses of focal lengths f1 and f2 are "thin", the combined focal length f of the lenses is given by,

1 / f = 1 / f1 + 1 / f2 − d / f1 x f2

If the ray is coming parallel to the incident parallel beam, then f = ∞

Therefore

120 + 15 − d / 100 = 0

⇒ 14 − d / 100 = 0

⇒ 25 − d / 400 = 0

⇒ d =25 cm

Thus the value of distance is 25 cm.

Also learn more

An object is placed 20cm. distance from the optical center of a convex lens. the focal length of the lens is 30cm. then calculate the object distance and mention the characteristics of the image formed ?

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