NUST NET (Engineering) · Physics · Modern and nuclear physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Three ideas changed physics in the early 1900s. First, special relativity: the speed of light is the same for every observer, so a moving clock runs slow, a moving object is shorter along its motion and its mass rises with speed. The effects matter only near the speed of light.
Second, light arrives in packets called photons, each of energy hf. One photon gives its energy to one electron, which explains the photoelectric effect: frequency decides the energy of the electrons, intensity decides only how many are emitted. The Compton effect shows that photons carry momentum too.
Third, particles behave as waves with a de Broglie wavelength, and position and momentum cannot both be known exactly.
Thinking brighter light gives faster photoelectrons. Intensity changes only the number emitted each second; the maximum kinetic energy depends on the frequency alone.
Answer: 1.5 eV (so the stopping potential is 1.5 V).
Try each one, then open the answer.
Answer: B. eV = p2/2m, so p ∝ √V and λ = h/p ∝ 1/√V. Four times V doubles p and halves λ. One-quarter wrongly takes λ ∝ 1/V.
Answer: D. The photon must supply the rest energy of two particles: 2m0c2 = 2 × 0.51 MeV = 1.02 MeV. 0.51 MeV is the rest energy of only one electron.
Answer: C. Kmax = hf − φ = 5.0 − 2.0 = 3.0 eV, and eV0 = Kmax, so V0 = 3.0 V. 7.0 V adds the work function instead of subtracting it.
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