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Physics · Unit 14

Dawn of Modern Physics: MDCAT Notes and MCQs

Photons

  • Planck: radiation is emitted/absorbed in discrete packets (quanta). Einstein: light itself consists of photons.
  • E = hf = hc/λ, h = 6.63 × 10−34 J s. Higher frequency (shorter λ) → more energetic photon (gamma > X-ray > UV > visible).
  • Photon properties: zero rest mass, travel at c, carry momentum p = h/λ = E/c, electrically neutral. Intensity of light ∝ number of photons (not their energy).
  • Energy in eV: E(eV) = 1240/λ(nm). 1 eV = 1.6 × 10−19 J.

Photoelectric effect — evidence for photons

  • Emission of electrons from a metal surface when light of suitable frequency falls on it.
  • Einstein's equation: hf = φ + K.E.max, where work function φ = hf0 (f0 = threshold frequency).
  • No emission below f0 however intense the light; KEmax depends on frequency not intensity; number of electrons (current) ∝ intensity; emission is instantaneous. Stopping potential: eV0 = KEmax.

Other evidence: Compton effect (X-ray scattering), pair production (E ≥ 1.02 MeV). Wave nature of particles: de Broglie λ = h/mv.

PMDC learning outcomes for this unit
  • Quantum Theory and Radiation: 14.1 Explain the particle model of light in terms of photons with energy

Dawn of Modern Physics MCQs with answers

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Q1
The energy of a photon is directly proportional to its:
ASpeed
BWavelength
CAmplitude
DFrequency
Show answer
Answer: D (Frequency). E = hf.
Q2
The value of Planck's constant is:
A1.6 × 10⁻¹⁹ J s
B6.63 × 10⁻³⁴ J s
C6.63 × 10³⁴ J s
D9.1 × 10⁻³¹ J s
Show answer
Answer: B (6.63 × 10⁻³⁴ J s). h = 6.63 × 10⁻³⁴ J s.
Q3
The momentum of a photon of wavelength λ is:
Aλ/h
Bhc/λ
Chλ
Dh/λ
Show answer
Answer: D (h/λ). p = h/λ.
Q4
In the photoelectric effect, the maximum KE of emitted electrons depends on:
AArea of surface
BIntensity of light
CTime of exposure
DFrequency of light
Show answer
Answer: D (Frequency of light). KEmax = hf − φ depends on frequency.
Q5
Increasing the intensity of light (above threshold frequency) increases the:
AWork function
BNumber of photoelectrons
CThreshold frequency
DKE of photoelectrons
Show answer
Answer: B (Number of photoelectrons). More photons per second release more electrons.
Q6
The rest mass of a photon is:
AInfinite
B9.1 × 10⁻³¹ kg
C1.67 × 10⁻²⁷ kg
DZero
Show answer
Answer: D (Zero). Photons have zero rest mass.
Q7
Which photon has the greatest energy?
ARed light
BBlue light
CInfrared
DRadio wave
Show answer
Answer: B (Blue light). Blue light has the highest frequency among these.
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