Free Quantum Theory and Radiation MCQs with Answers

• A. continuously in any amount
• B. in discrete packets called quanta, each of energy hf
• C. only as heat
• D. only by solids

Explanation: Planck found that black body radiation could only be explained if the energy came in multiples of hf, where h is Planck's constant and f…

• A. hf
• B. h divided by f
• C. hc
• D. mc squared only

Explanation: Energy is Planck's constant multiplied by frequency, equivalently hc divided by wavelength, so shorter wavelength means more energetic…

• A. electrons are slowed down suddenly on striking a metal target
• B. a metal is heated to a very high temperature
• C. light is passed through a prism
• D. a nucleus decays

Explanation: Fast electrons decelerating in the target emit a continuous spectrum of X rays known as bremsstrahlung, and knocking out inner shell…

• A. reflects all radiation falling on it
• B. absorbs all radiation falling on it and is also the best possible emitter
• C. emits no radiation at all
• D. is black in colour at all temperatures

Explanation: A perfect absorber must also be a perfect emitter at the same temperature, otherwise it could not stay in thermal equilibrium with its…

• A. gets longer
• B. gets shorter
• C. stays the same
• D. becomes infinite

Explanation: Wien's displacement law says the peak wavelength is inversely proportional to absolute temperature, which is why heated iron glows dull…

• A. 2 eV
• B. 20 eV
• C. 0.2 eV
• D. 200 eV

Explanation: Using the convenient result that photon energy in electron volts is about 1240 divided by the wavelength in nanometres, 1240 over 600…

• A. decreases
• B. increases relative to an observer at rest
• C. stays exactly the same
• D. becomes zero

Explanation: The relativistic factor grows without limit as the speed approaches c, so the effective mass rises and an infinite force would be needed…

• A. mass and energy are different names for one physical quantity that can be converted between forms
• B. mass can never be converted to energy
• C. energy has no mass
• D. c is a variable

Explanation: Because c squared is so large, a tiny loss of mass releases an enormous quantity of energy, which is exactly what happens in nuclear…

• A. Green
• B. White
• C. Blue
• D. Black

Explanation: If a black body emits all visible colours with suitable intensities, their additive combination is perceived as white light.

• A. Dilates
• B. Contracts
• C. Constant
• D. Uniform

Explanation: Special relativity predicts time dilation: a clock moving relative to an observer is measured to run more slowly, so the time interval…

• A. E=mc²
• B. E=hf
• C. E=m²c
• D. E=mv²

Explanation: Einstein's mass-energy relation states that the energy equivalent of mass m is E = mc², where c is the speed of light in m/s.

• A. Exude
• B. Reflect
• C. Absorb
• D. None of these

Explanation: A black body is an ideal absorber that absorbs all incident electromagnetic radiation, regardless of wavelength or direction.

• A. Potential energy
• B. Rest energy
• C. Chemical energy
• D. Nuclear energy

Explanation: Matter has rest energy because a body of mass m possesses energy E = mc² even when it is not moving.

• A. Quantum theory
• B. General theory of relativity
• C. Special theory of relativity
• D. None of these

Explanation: The relation E = mc² follows from special relativity, which treats mass and energy as equivalent forms of the same physical quantity.

• A. G
• B. h
• C. M
• D. E

Explanation: The prefix mega represents 10^6 and its SI symbol is capital M. The likely mistake is choosing G, which is the symbol for giga and…

• A. protons
• B. positrons
• C. electrons
• D. photons

Explanation: Thermions are electrons emitted from a heated metal surface when thermal energy gives them enough energy to overcome the surface potential…

• A. 10-4 sec
• B. 10-5 sec
• C. 10-3 sec
• D. 10-8 sec

Explanation: A metastable state has an unusually long lifetime, typically about 10^-3 s, compared with an ordinary excited state of roughly 10^-8 s…

• A. Metal stable state
• B. Normal state
• C. Inverted population
• D. All the above

Explanation: When more atoms occupy the upper energy level than the lower level, the condition is called population inversion, which is necessary for…

• A. Stimulated
• B. Absorbed
• C. Cause X-ray emission
• D. Cause laser production

Explanation: In the normal energy state, most atoms are in the lower level, so an incident photon with the correct energy is absorbed to raise an atom…

• A. It does emit radiation
• B. Its size as compare to proton and neutron is very small
• C. No antiparticle of electron is present
• D. The velocity of electron must be very high according to uncertainty principle

Explanation: If an electron were confined inside a nucleus of extremely small size, the uncertainty principle would require a very large uncertainty in…