Free Planck's Quantum Theory MCQs with Answers

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

• A. continuously in any amount
• B. in discrete packets whose energy is proportional to frequency
• C. only as heat
• D. in packets whose energy is proportional to wavelength

Explanation: Planck proposed that radiation is emitted in quanta of energy E equal to h times the frequency, where h is Planck's constant. Since frequency and wavelength are inversely related, energy is inversely proportional to wavelength, which is why the last option is wrong rather than merely imprecise. The quantum idea was what resolved the black body radiation problem that classical physics could not.

2. Planck's constant has the value

• A. 6.63 times 10 to the minus 34 J s
• B. 3.0 times 10 to the 8 J s
• C. 1.6 times 10 to the minus 19 J s
• D. 9.1 times 10 to the minus 31 J s

Explanation: The extreme smallness of Planck's constant is why quantisation is invisible in everyday life and only matters at atomic scale. The other values given are the speed of light, the electronic charge and the electron mass respectively, all of which appear alongside it in the same formulae. Knowing which constant is which prevents easy marks being dropped.

3. The energy of a quantum of radiation is directly proportional to its

• A. wavelength
• B. frequency
• C. speed
• D. amplitude

Explanation: Energy equals Planck's constant multiplied by frequency, so higher frequency means more energetic quanta, and since frequency and wavelength are inversely related, energy falls as wavelength rises. This is why ultraviolet is damaging and infrared is not. Amplitude governs the number of photons, that is the intensity, rather than the energy of each.

4. The value of Planck's constant is:

• A. 6.6262x10-30 J.sec
• B. 6.6262x10-32 J.sec
• C. 6.6262x10+34 J.sec
• D. 6.6262x10-34 J.sec

Explanation: Planck's constant is 6.626 times 10 to the minus 34 joule seconds, and its extreme smallness is why quantisation is invisible in everyday life and only matters at atomic scale. The positive exponent in one option would make it enormous, which a moment's thought rules out. It appears in E equals hf, in the de Broglie relation and in the uncertainty principle.