Convex lens in water: focal length: MCQ with Answer and Explanation

Convex lens in water: focal length:
A. Unchanged
B. Becomes negative
C. Decreases
D. Increases
Answer: Option D
Solution (By JKSSB Mock Tests)
Lens maker: 1/f ∝ (n_rel - 1). In water, n_rel = n_glass/n_water ≈ 1.5/1.33 ≈ 1.13 < 1.5 (air). Smaller (n_rel-1) ⇒ larger f. Memory aid: 'Lens in denser medium: reduced power ⇒ longer focal length'. Optics application frequently tested in competitive exams.

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Practice More Physics Questions

Question #1
The least count of a screw gauge is 0.001 cm. The reading of a measurement is 0.250 cm. How many significant figures does it have?
A. 3
B. 2
C. 4
D. 1

Correct Answer: Option A


Explanation:
Reading 0.250 cm has three significant figures (2,5, and trailing zero after decimal). Least count gives precision, trailing zero indicates measurement to 0.001 cm, so it's significant.

This question belongs to: Science Physics
Question #2
The frequency of a wave is 1 kHz. Its time period is
A. 1000 s
B. 1 s
C. 0.01 s
D. 0.001 s

Correct Answer: Option D


Explanation:
T = 1/f = 1/1000 = 0.001 s = 1 ms.

This question belongs to: Science Physics
Question #3
The device that protects electrical circuits from overload is
A. Fuse
B. Resistor
C. Switch
D. Capacitor

Correct Answer: Option A


Explanation:
Fuse melts and breaks circuit when current exceeds rating. MCB (Miniature Circuit Breaker) does same but resettable. Switch only turns on/off. Resistor limits current but doesn't break.

This question belongs to: Science Physics