Lens power P = -4 D. Focal length and type: MCQ with Answer and Explanation

Lens power P = -4 D. Focal length and type:

A. 40 cm, concave

B. 40 cm, convex

C. 25 cm, concave

D. 25 cm, convex

Answer: Option C

Solution (By JKExamLibrary)

P = 1/f(m) ⇒ f = 1/P = 1/(-4) = -0.25 m = -25 cm. Negative focal length indicates diverging (concave) lens. Memory aid: 'Negative power = concave lens; f(cm) = 100/P(D)'. Direct lens power application frequently tested in competitive exams with sign convention awareness.

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Question #1 Report Error

If a stone is dropped from a balloon rising upwards with a velocity of 10 m/s at a height of 75 m, what is the initial velocity of the stone relative to the ground?

A. 0 m/s

B. 9.8 m/s downwards

C. 10 m/s downwards

D. 10 m/s upwards

View Answer & Solution Discuss

Correct Answer: Option D

Explanation:
Due to the inertia of motion, when an object is dropped from a moving vehicle (or balloon), it inherits the instantaneous velocity of that vehicle. Since the balloon was moving upwards at 10 m/s, the stone's initial velocity relative to the ground is 10 m/s upwards. It will rise slightly before falling.

This question belongs to: Science Physics