Free Quantum Theory and Radiation MCQs with Answers

1. Planck's quantum theory proposed that energy is emitted or absorbed

• 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 the frequency. Classical physics assumed a continuous exchange and predicted infinite energy at short wavelengths, the so called ultraviolet catastrophe. Quantisation resolved that failure and began quantum physics.

2. The energy of a photon is given by

• 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 photons. This is why ultraviolet light damages skin while visible light of the same intensity does not. Planck's constant is 6.63 times 10 to the minus 34 joule seconds.

3. X rays are produced when

• 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 electrons produces sharp characteristic lines on top of it. The efficiency is low, so most of the electron energy becomes heat and the target must be cooled. Gamma rays are similar radiation but originate in the nucleus instead.

4. A black body is one that

• 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 surroundings. The spectrum of the radiation it emits depends only on temperature, which is why the concept is so useful for stars. A small hole in a hollow cavity is the standard practical approximation.

5. As a black body gets hotter, the wavelength at which it radiates most strongly

• 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 red and then white as it gets hotter. The same law lets astronomers deduce a star's surface temperature from its colour. The total power radiated rises even faster, as the fourth power of temperature.

6. The photon energy of light of wavelength 600 nm is closest to

• 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 gives roughly 2 eV. Visible photons all lie between about 1.6 and 3.1 eV, which is a useful range to remember as a check. Values of hundreds of electron volts belong to X rays.

7. According to Einstein's theory of special relativity, the mass of an object moving at a speed close to that of light

• 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 to reach the speed of light, which is why no massive object can attain it. The effect is negligible at everyday speeds, which is why Newtonian mechanics works so well. Length contraction and time dilation follow from the same factor.

8. The mass energy relation E equals mc squared implies that

• 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 fission and fusion. The mass defect of a nucleus, converted by this relation, gives its binding energy. Chemical reactions release energy too, but the accompanying mass change is far too small to measure.