PHYSICS CONNECTION
LED voltage links electrical and photon energy
The energy transferred per electron by an LED’s threshold voltage can be compared with the energy of emitted photons. Different colours give a route to estimating Planck’s constant.
eV ≈ hf
Follow the reasoning
- Measure a consistent threshold voltage for each LED.
- Use the frequency of its emitted light.
- Plot voltage against frequency and relate the gradient to h/e.
OFFICIAL OCR QUESTIONS
Try it in a past paper 2
Open the question, then use its mark scheme to check your work.
Question 8
Use LED measurements and a graph to estimate Planck’s constant and its uncertainty.
Question 12
Relate LED colour to the potential difference needed for light emission.
A curated collection checked against the question papers and mark schemes; not a complete past-paper catalogue. Links open official OCR PDFs in a new tab.
ORIGINAL PHYSICSUK QUESTIONS
Put the ideas to work
More practice is on its way
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Specification outcomes (4)
- 4.2.2(d) Use energy transfer equations W = VQ and W = EQ.
- 4.5.1(c) Use E = hf and E = hc / λ to calculate energy of a photon.
- 4.5.1(e)(i) Use LEDs and eV = hc / λ to estimate the value of Planck’s constant.
- 4.5.1(e)(ii) Determine Planck’s constant using different coloured LEDs experimentally.