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

Convex lens in water: focal length:
A. Becomes negative
B. Decreases
C. Increases
D. Unchanged
Answer: Option C
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
A passenger in a moving train tosses a coin which falls behind him. This shows that the motion of the train is:
A. Uniform
B. Retarded
C. Accelerated
D. Along a circular track

Correct Answer: Option C


Explanation:
When the coin is tossed, it moves forward with the initial velocity of the train due to inertia of motion. If the train accelerates during the time the coin is in the air, the train moves faster and travels a greater distance horizontally than the coin. Thus, the coin falls behind the passenger.

This question belongs to: Science Physics
Question #2
A body of weight 20 N is immersed in water, apparent loss in weight is 5 N. Buoyant force is
A. 5 N
B. 15 N
C. 20 N
D. 25 N

Correct Answer: Option A


Explanation:
Apparent loss in weight = buoyant force = weight in air - weight in water = 5 N. Archimedes principle. Apparent weight = 20-5 = 15 N.

This question belongs to: Science Physics
Question #3
An echo is heard from a cliff after 3 s. If speed of sound is 340 m/s, distance of cliff is
A. 340 m
B. 510 m
C. 1020 m
D. 170 m

Correct Answer: Option B


Explanation:
d = v × t / 2 = 340 × 3 / 2 = 510 m.

This question belongs to: Science Physics