IGCSE · 3 · extended response
Same light energy, different photon counts
Waves · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 6
- Topics
- 1
- Answer
- Complete
Photon-energy model
Supplied question data. Read the labelled axes and use the observations to test the physical model.
Vertical axis (y): Photon energy / 10⁻¹⁹ J
Horizontal axis (x): Frequency / 10¹⁴ Hz
Read the exact data as a table
| Frequency / 10¹⁴ Hz | Photon energy / 10⁻¹⁹ J |
|---|---|
| 0 | 0 |
| 5 | 3.313 |
| 10 | 6.626 |
These are the supplied values, not a worked answer. Lines join the supplied values; they are not a fitted model.
Optional enrichment: this challenge may extend beyond the usual syllabus depth and assumes confidence with algebra and graph interpretation. A source emits monochromatic pulses of equal energy 6.626×10⁻⁶ J at two frequencies: 5.0×10¹⁴ Hz and 1.0×10¹⁵ Hz. Use h = 6.626×10⁻³⁴ J s and c = 3.0×10⁸ m s⁻¹. The graph provides photon energy as a function of frequency. Supplied model: Use photon energy E = hf, wavelength λ = c/f and photon count N = Epulse/Ephoton. Supplied data (Frequency / 10¹⁴ Hz; Photon energy / 10⁻¹⁹ J): (0, 0), (5, 3.313), (10, 6.626).
- (a)
Calculate each wavelength, photon energy and photon count. Identify the approximate spectral region of each pulse.
3 marks - (b)
Explain why interference supports a wave model while photon-counting detection needs a particle-like model. Evaluate the claim that all electromagnetic radiation has identical biological effects.
3 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
Calculate each wavelength, photon energy and photon count. Identify the approximate spectral region of each pulse.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
Use λ = c/f: the lower frequency has wavelength 6.0×10⁻⁷ m and the higher 3.0×10⁻⁷ m.
- 2
Ephoton = hf gives 3.313×10⁻¹⁹ and 6.626×10⁻¹⁹ J. Dividing equal total pulse energy by these values gives 2.0×10¹³ and 1.0×10¹³ photons.
- 3
600 nm is visible light and 300 nm is ultraviolet. Both propagate at c in vacuum; the higher-frequency pulse contains fewer photons for the same total energy.
(b)
Explain why interference supports a wave model while photon-counting detection needs a particle-like model. Evaluate the claim that all electromagnetic radiation has identical biological effects.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
Spatial interference patterns follow phase and superposition, which a simple stream of classical particles does not explain.
- 2
Localized detector events with frequency-dependent energy are described by photons. These are complementary descriptions of quantum light rather than literal tiny balls riding classical waves.
- 3
Absorption, penetration, dose and photon energy depend on wavelength and material. Ultraviolet can cause photochemical damage; exposure assessment cannot be inferred from vacuum speed alone.
Complete marking guidance
Use these worked steps alongside the part-by-part answers. They explain the method and mark allocation; they do not add extra marks.
- Use λ = c/f: the lower frequency has wavelength 6.0×10⁻⁷ m and the higher 3.0×10⁻⁷ m.
- Ephoton = hf gives 3.313×10⁻¹⁹ and 6.626×10⁻¹⁹ J. Dividing equal total pulse energy by these values gives 2.0×10¹³ and 1.0×10¹³ photons.
- 600 nm is visible light and 300 nm is ultraviolet. Both propagate at c in vacuum; the higher-frequency pulse contains fewer photons for the same total energy.
- Spatial interference patterns follow phase and superposition, which a simple stream of classical particles does not explain.
- Localized detector events with frequency-dependent energy are described by photons. These are complementary descriptions of quantum light rather than literal tiny balls riding classical waves.
- Absorption, penetration, dose and photon energy depend on wavelength and material. Ultraviolet can cause photochemical damage; exposure assessment cannot be inferred from vacuum speed alone.
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