Below threshold
What to notice
A brighter beam sends more photons, but it does not increase the energy carried by each photon.
Photon energy laboratory
PhotoelectricEffect
Shine light on a metal and release electrons—but only when each photon carries enough energy to cross the material's energy barrier.
1Choose light2Test barrier3Collect
Can electrons escape?
First change wavelength to cross the threshold. Then change intensity and compare what happens.
Metal surfaceSimplified work-function values
Wavelength λShorter wavelength means more photon energy
UVVisibleIR
Light intensityControls photons per second
Photon energy2.07 eV
Max kinetic energy0.07 eV
Threshold λ620 nm
Electrons collected0
Just above the barrier2.07 eV − 2.00 eV = 0.07 eV kinetic energy
Energy comes in photons
Each photon carries energy set by its frequency, or equivalently by its wavelength.
Ephoton = hf = hc / λ
There is a threshold
Below the work function φ, no electrons escape—no matter how intense the light becomes.
Kmax = hf − φ
Intensity changes quantity
Above threshold, brighter light releases more electrons. Their maximum energy stays set by wavelength.
Challenge
Explain with AI →Make electrons escape from the 4.3 eV surface.
Does increasing intensity work, or must you change the wavelength?