Meaning and mechanism
LED is a device that produces light by recombination of electrons and holes within a semiconductor. The energy of the emitted light is related to the material’s band structure.
Shorter wavelengths of light correspond to higher frequencies. We will compare the voltages corresponding to the current or emission standards set for multiple colored LEDs to examine the energy relationship.
Relationships and conditions
Using f=c/λ and the educational approximation eVth≈hf, multiplying the slope of Vth against f by e gives an estimate of h. The trend across several LEDs matters more than one LED alone.
The threshold is influenced by the detection current, the sensitivity of the eye, and junction voltage, among other factors. In reality, the electrical energy in a real LED does not all convert into light with the same energy.
What to observe in the simulation
Higher-frequency LEDs have higher threshold voltages. Estimate h from the slope using several colors.
This is an educational approximation eV ≈ hf. Real LED band gaps, contact voltages and turn-on criteria can cause larger errors. Noise is a reproducible fixed color correction. The threshold button helps locate the ideal model boundary. Use this simulation; do not reproduce it with real power supplies or wall outlets.
Key relationships and symbols
f = c/λ · eVth ≈ hf · h estimate = e × slope(Vth vs f)
| Symbol | Meaning | Unit |
|---|---|---|
| λ, f | Wavelength and frequency of light | m, Hz |
| h | Planck’s constant | J·s |
| e | Magnitude of the elementary charge | C |
Worked example
If the V–f slope is 4.14×10⁻¹⁵ V·s, what estimate of h results from multiplying by e?
About 6.63×10⁻³⁴ J·s
Common misconceptions
- Asserting that the threshold voltage of an LED is exactly equal to the photon energy is an interpretation that disregards approximation and measurement errors.