A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed the point where the rays meet will move 5 cm closer to the lens. The focal length of the lens is

Question

Dr. Abubacker · Accepted Answer

[Solution from source] Without the lens the rays meet 10 cm behind the lens position, so that point is a virtual object at u = +10 cm; with the lens the image is at v = +15 cm. 1/f = 1/v − 1/u = 1/15 − 1/10 = −1/30, so f = −30 cm. The negative sign confirms a diverging lens, ruling out the positive values. Answer (1).

Q. A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed the point where the rays meet will move 5 cm closer to the lens. The focal length of the lens is

Explanation

Reviewed by Dr. Abubacker

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