Free Photons and the Particle Model of Light MCQs with Answers

• 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. 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…

• A. Moving with speed of light
• B. At rest
• C. In accelerated form
• D. Moving with speed of sound

Explanation: A photon is a quantum of electromagnetic radiation and travels at 3.0 x 10^8 m/s in vacuum, the speed of light.

• A. Mosely
• B. Marley
• C. Arthur Holly Compton
• D. Stefan Boltzman

Explanation: Arthur Holly Compton explained the scattering of X-rays by electrons using the photon model and showed that the scattered wavelength…

• A. Two
• B. Three
• C. Four
• D. One

Explanation: When an electron and positron annihilate at rest, their rest energy is converted into two gamma-ray photons moving in opposite directions…

• A. Photon
• B. Meson
• C. Vector Boson
• D. Graviton

Explanation: The electromagnetic interaction is carried by photons, which are massless quantum particles of the electromagnetic field.

• A. Number of incident electron i.e. filament current
• B. The kinetic energy of the incident electron i.e. the potential difference through which they are accelerated
• C. The soft target which can easily emit electrons
• D. All of above are correct

Explanation: The maximum X-ray frequency follows h f_max = eV, so f_max = eV/h and is directly proportional to the accelerating potential difference…

• A. 6.2 x 10^-10m
• B. 9.1 x 10^-18m
• C. 3.11 x 10^-10m
• D. 4 x 10^-10m

Explanation: The minimum wavelength is λ_min = hc/eV = (6.63 x 10^-34 J s)(3.0 x 10^8 m/s)/(1.60 x 10^-19 C)(2000 V) = 6.2 x 10^-10 m.

• A. Reducing the pressure or cooling the target
• B. Increasing the temperature of the filament.
• C. Using a target element of higher atomic number
• D. Increasing the potential difference between the cathode and the target

Explanation: Since λ_min = hc/eV, increasing the potential difference V increases the electron energy and decreases the minimum X-ray wavelength.

• A. B-rays
• B. X-rays
• C. Far infrared
• D. Infrared

Explanation: Electrons accelerated through the high voltage in a television picture tube can strike the screen and produce bremsstrahlung X-rays…