Unit 2 — Introduction to Modern Physics · Physics. These are 10 questions from the Magmo3 bank on this lesson, from easy to hard. Solve each one, then open "Show answer and explanation". Magmo3 has 20 questions on this lesson, each with an explanation.
Laser light is said to be coherent because its emitted photons...
Correct answer: (A) travel in phase with one another
Explanation: Coherent laser photons keep a constant phase relationship (they travel in phase).
The process in which an incident photon causes an excited atom to emit a second photon identical to the incident one is known as...
Correct answer: (C) stimulated emission
Explanation: In stimulated emission an incident photon triggers an excited atom to release an identical (coherent) photon.
A laser beam is described as monochromatic because all of its photons have the same...
Correct answer: (B) wavelength
Explanation: Monochromatic light consists of a single wavelength (one colour).
Population inversion in the active medium of a laser is normally achieved by a process called...
Correct answer: (D) pumping energy into the medium
Explanation: Pumping (optical or electrical) lifts atoms to the higher level, producing population inversion.
In a laser tube, the pair of parallel reflecting mirrors placed at the two ends of the active medium mainly serves to...
Correct answer: (C) increase the number of photons by repeated stimulated emission
Explanation: The mirrors reflect photons back and forth so that repeated stimulated emission amplifies the beam.
A laser beam can travel a great distance while remaining narrow mainly because it has a very small...
Correct answer: (C) angle of divergence
Explanation: The very small angle of divergence keeps the laser beam narrow over long distances.
In a helium-neon laser, the neon atoms are raised to their metastable level mainly through...
Correct answer: (C) inelastic collisions between excited helium atoms and neon atoms
Explanation: Excited helium atoms transfer their energy to neon atoms by inelastic collisions, populating the neon metastable level.
The momentum of each photon in a laser beam of wavelength 6.6×10⁻⁷ metre is (h = 6.6×10⁻³⁴ J·s)...
Correct answer: (C) 1×10⁻²⁷ kg·m/s
Explanation: p = h/λ = 6.6×10⁻³⁴ ÷ 6.6×10⁻⁷ = 1×10⁻²⁷ kg·m/s.
In a helium-neon laser, excited helium atoms (level 20.61 eV) collide with neon atoms and raise them to a metastable level of 20.66 eV. The laser photons are emitted when neon atoms drop from this metastable level to a level of 18.70 eV. The wavelength of the laser light is (hc = 1240 eV·nm)…
Correct answer: (A) 633 nm
Explanation: Only the neon transition produces the laser photon: E = 20.66 − 18.70 = 1.96 eV, so λ = 1240/1.96 ≈ 633 nm (red). The 0.05 eV gap between helium and neon levels is not a laser transition.
A laser emits a continuous beam of power 6.6 mW and wavelength 660 nm. The number of photons emitted per second is (h = 6.6×10⁻³⁴ J·s, c = 3×10⁸ m/s)...
Correct answer: (C) 2.2×10¹⁶ photons/s
Explanation: N = P ÷ (hc/λ) = 6.6×10⁻³ ÷ 3×10⁻¹⁹ = 2.2×10¹⁶ photons per second.
The first 10 questions in every subject are free, each with an explanation.