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

Convex lens in water: focal length:
A. Decreases
B. Becomes negative
C. Unchanged
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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Question #1
What is the magnetic dipole moment of an electron revolving in a circular orbit of radius 'r' with a uniform speed 'v'?
A. e / (vr)
B. evr
C. 2evr
D. evr / 2

Correct Answer: Option D


Explanation:
Magnetic dipole moment M = Current(I) * Area(A). Current is charge/time: I = e / T. Time period T = 2pir / v. So I = ev / (2pir). The area of the circular orbit is A = pir^2. Therefore, M = [ev / (2pir)] * [pir^2] = evr / 2. This is a standard derivation in modern physics.

This question belongs to: Science Physics
Question #2
Newton's first law defines:
A. Force
B. Momentum
C. Acceleration
D. Inertia

Correct Answer: Option D


Explanation:
Newton's first law (law of inertia) states bodies remain at rest or uniform motion unless acted upon by external force. This defines inertia - resistance to change in motion state. Force is defined in second law; momentum conservation in third. Memory aid: 'First law = inertia; second = F=ma; third = action-reaction'. Fundamental concept frequently tested to assess basic mechanics understanding.

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Question #3
Nuclear reactor moderator function:
A. Slow neutrons
B. Shield radiation
C. Emit neutrons
D. Absorb neutrons

Correct Answer: Option A


Explanation:
Moderator (water, graphite) slows fast neutrons to thermal energies for efficient fission in U-235. Control rods absorb neutrons to regulate reaction. Memory tip: 'Moderator = neutron slower; Control rods = neutron absorber'. Reactor component function frequently tested in competitive nuclear physics questions.

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