Nuclear and quantum physics practice questions.

Which type of ionising radiation is the most penetrating?

• alpha
• beta
• gamma
• they are equal

Answer: gamma

Gamma is the most penetrating (needs thick lead/concrete). Alpha is stopped by paper; beta by a few mm of aluminium.

Light of wavelength 500 nm strikes a metal of work function 2.0 eV. What is the maximum kinetic energy of emitted electrons? (hc = 1240 eV·nm)

• 0.48 eV
• 2.48 eV
• 4.48 eV
• 1.24 eV

Answer: 0.48 eV

Photon energy = 1240/500 = 2.48 eV. E_max = 2.48 − 2.0 = 0.48 eV.

A radioactive sample decays according to N = N₀e^(−λt). After exactly 3 half-lives, what fraction of the original nuclei remain?

• 1/3
• 1/6
• 1/8
• 1/9

Answer: 1/8

Each half-life halves the number: (1/2)³ = 1/8 remain.

Which type of ionising radiation is stopped by a single sheet of paper?

• alpha
• beta
• gamma
• X-rays

Answer: alpha

Alpha particles are the most strongly ionising and least penetrating; a sheet of paper (or a few cm of air) stops them. Beta needs a few mm of aluminium; gamma needs thick lead or concrete.

Monochromatic light below the threshold frequency shines on a metal surface. Increasing the intensity of this light will

• cause electrons to be emitted
• increase the kinetic energy of emitted electrons
• still cause no electrons to be emitted
• decrease the work function of the metal

Answer: still cause no electrons to be emitted

Below the threshold frequency each photon carries too little energy (E = hf) to release an electron – intensity only changes the number of photons, not their energy.

According to the de Broglie hypothesis, the wavelength associated with a moving particle is inversely proportional to its

• mass only
• momentum
• kinetic energy
• electric charge

Answer: momentum

λ = h/p, so the de Broglie wavelength is inversely proportional to the particle's momentum p = mv.