The phenomenon of photoelectric emission occurs only when incident light has: MCQ with Answer and Explanation

Question
The phenomenon of photoelectric emission occurs only when incident light has:
A. Long wavelength
B. Frequency above threshold
C. Any frequency if intensity is high
D. High intensity
Answer: Option B
Solution (By JKSSB Mock Tests)
Photoelectric effect requires photon energy hν ≥ work function φ of metal. Thus frequency ν must exceed threshold ν₀ = φ/h, regardless of intensity. Intensity affects number of electrons, not emission possibility. Memory aid: 'Photoelectric: frequency threshold (quantum effect); intensity affects current, not emission'. This quantum physics concept is frequently tested in competitive exams. Always distinguish classical wave prediction (intensity-dependent) from quantum reality (frequency-dependent); this was key evidence for photons.
This question belongs to: Science Physics
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Question #1
The color of sky is blue due to
A. Scattering
B. Refraction
C. Reflection
D. Dispersion
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Correct Answer: Option A

Explanation:
Rayleigh scattering of sunlight by air molecules, blue scattered most.
This question belongs to: Science Physics
Question #2
Which scientific instrument is used specifically for measuring very high temperatures, such as the surface temperature of the Sun or inside a furnace, from a distance?
A. Bolometer
B. Cryometer
C. Calorimeter
D. Pyrometer
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Correct Answer: Option D

Explanation:
A pyrometer is a non-contact thermometer used to measure high temperatures. It operates on the principle of thermal radiation (Stefan-Boltzmann law or Wien's law), measuring the intensity or spectrum of electromagnetic radiation emitted by the extremely hot object. A cryometer measures very low temperatures, and a calorimeter measures heat capacity.
This question belongs to: Science Physics
Question #3
According to the Equation of Continuity in fluid dynamics, when water flows through a pipe of varying cross-section, the velocity of the fluid is:
A. Directly proportional to the square of the cross-sectional area.
B. Directly proportional to the cross-sectional area.
C. Independent of the cross-sectional area.
D. Inversely proportional to the cross-sectional area.
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Correct Answer: Option D

Explanation:
The Equation of Continuity states that for an incompressible, non-viscous fluid in streamlined flow, the mass flow rate is constant. Therefore, A1V1 = A2V2, where A is the area of cross-section and V is the velocity. This means V is inversely proportional to A (V ∝ 1/A). As the pipe narrows, the fluid speeds up.
This question belongs to: Science Physics
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