STEP 1 · OBSERVE
What happens when light strikes a metal?
Light arrives as photons. If an individual photon has enough energy to overcome the metal's work function, an electron can escape from the surface.
Explore the light
200 nm · UV700 nm · Red
LowerHigher
Photon energy3.10 eV
Work function, φ2.28 eV
Maximum kinetic energy0.82 eV
Stopping potential0.82 V
The photon energy exceeds the work function, so photoemission is possible.
GRAPHICAL ANALYSIS
Finding Planck's constant from the photoelectric effect
Einstein's equation predicts a straight-line relationship between maximum kinetic energy and frequency: Kmax = hν − φ. The gradient gives Planck's constant when the axes are expressed in SI units.
Gradient, ΔKmax/Δν4.136 × 10⁻¹⁵ eV·s
Planck's constant, h6.626 × 10⁻³⁴ J·s
Threshold frequency, ν₀5.51 × 10¹⁴ Hz
The line's gradient represents Planck's constant. The horizontal-axis intercept gives the threshold frequency for the selected metal.
How to read the graph
- Straight-line gradient: Planck's constant, h.
- Frequency-axis intercept: threshold frequency, ν₀ = φ/h.
- Vertical intercept: −φ, the negative work function.