Free Dawn of Modern Physics MCQs with Answers

11. In the photoelectric effect, the stopping potential is the voltage that

• A. just prevents the most energetic photoelectrons from reaching the collector
• B. starts the emission of electrons
• C. doubles the photocurrent
• D. equals the work function in volts

Explanation: Applying a retarding voltage reduces the photocurrent, and the value that cuts it to zero measures the maximum kinetic energy directly, since that energy equals the electronic charge multiplied by the stopping potential. The stopping potential depends on frequency but not on intensity. Plotting it against frequency is the classic experiment for finding Planck's constant.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. Which observation cannot be explained by treating light purely as a wave?

• A. Interference
• B. Diffraction
• C. Polarisation
• D. The existence of a threshold frequency in the photoelectric effect

Explanation: A wave of any frequency should eventually deliver enough energy if it shines long enough or brightly enough, so the sharp frequency threshold makes no sense on the wave picture and demands photons. Interference, diffraction and polarisation, by contrast, are precisely the phenomena that establish the wave nature. Light therefore needs both descriptions.

19. The momentum of a photon of wavelength lambda is

• A. h divided by lambda
• B. h times lambda
• C. zero, since a photon has no mass
• D. mc

Explanation: A photon has zero rest mass but still carries momentum h over lambda, which is what allows radiation pressure and the recoil seen in the Compton effect. Solar sails are propelled by exactly this momentum transfer. Assuming that no rest mass means no momentum is the standard misconception here.

20. The photoelectric effect is best demonstrated with which metal, given that it has a very low work function?

• A. Caesium
• B. Iron
• C. Tungsten
• D. Platinum

Explanation: Caesium's work function is only about 2 eV, so ordinary visible light is enough to eject electrons, whereas tungsten and platinum need ultraviolet. This is why alkali metals are used in photocells and photomultipliers. The low value reflects how loosely the single outer electron of an alkali metal is held.