Explain why a dim source of light with a high enough frequency can release electrons from a metal surface, but a very bright source of light with a lower frequency cannot.

Light is quantised – it consists of photons.

The energy of each photon is given by E=hf, where h is Planck’s constant and f is the frequency of the light. The intensity (brightness) of the light only tells how many photons reach the surface per unit time; it does not change the energy of each photon.

For an electron to be emitted from a metal, a photon must supply at least the work function (ϕ) of that metal. If hf>ϕ, the electron can be released; if hf<ϕ, no electron can be ejected, no matter how many photons arrive.

Thus a dim source of high‑frequency light contains photons whose energy exceeds the work function, so electrons are emitted. A bright source of low‑frequency light contains photons whose energy is below the work function, so no electrons are emitted, even though the intensity is high.