Free Dawn of Modern Physics 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. sufficient intensity, whatever its frequency
• B. a frequency above a threshold value characteristic of the metal
• C. been shone for long enough
• D. a long wavelength

Explanation: Below the threshold frequency no electrons are emitted however bright the light or however long it shines, which the wave theory could not…

• A. the maximum kinetic energy of the emitted electrons
• B. the number of electrons emitted per second
• C. the threshold frequency
• D. the work function of the metal

Explanation: More intense light means more photons per second, so more electrons are ejected, but each photon still carries the same energy hf and so…

• A. hf plus the work function
• B. hf minus the work function
• C. the work function minus hf
• D. hf multiplied by the work function

Explanation: The photon energy hf is spent first on freeing the electron from the metal, which costs the work function, and whatever remains appears as…

• A. the minimum energy needed to remove an electron from its surface
• B. the energy of the incident photon
• C. the kinetic energy of the fastest emitted electron
• D. the total energy of all the electrons

Explanation: Work function is a property of the metal, usually quoted in electron volts, and metals such as caesium with low values emit electrons even…

• A. have no momentum
• B. carry momentum and collide like particles
• C. travel slower than light
• D. have mass at rest

Explanation: The scattered photon gives up part of its energy and momentum to the recoiling electron, so its frequency falls and its wavelength rises…

• A. h divided by its momentum
• B. h multiplied by its momentum
• C. hf
• D. c divided by its speed

Explanation: Wavelength is Planck's constant divided by mv, so heavy everyday objects have wavelengths far too small to detect while an electron's is…

• A. electrons travel faster than light
• B. the de Broglie wavelength of a fast electron is far shorter than the wavelength of visible light
• C. electrons are smaller than photons
• D. it uses a stronger lens

Explanation: Resolution is limited by wavelength, and accelerating electrons to high speed gives them a wavelength of the order of a picometre…

• A. light is a wave and matter is a particle, always
• B. light and matter each show wave behaviour in some experiments and particle behaviour in others
• C. waves and particles are the same thing in every respect
• D. particles travel in waves through a medium

Explanation: Interference and diffraction reveal the wave nature of light, while the photoelectric and Compton effects reveal its particle nature, and…

• 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…

• 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. 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…

• 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…

• 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…