To escape from a metal, an electron must be given a minimum amount of energy called the work function. In the photoelectric effect, this energy comes from a single photon striking a single electron, so what matters is the energy of each individual photon.
Photon energy depends on frequency. Red light has a relatively low frequency, so each of its photons carries less than the work function of most metals. That one photon simply cannot give the electron enough energy to leave. Turning up the brightness sends many more red photons, but each is still too weak, and photons do not team up on one electron, so no electrons are emitted at all.
Switch to a high-frequency light such as ultraviolet, and each photon now carries enough energy, so electrons come out immediately, even if the light is dim.
In SPM you should use the threshold frequency and the work function to explain why frequency, not brightness, controls the effect.
Common misconceptions
- Enough red photons can add their energy to free one electron -> Each electron absorbs a single photon; weak photons do not combine on one electron.
- Brightness can always compensate for low frequency -> Below the threshold frequency, no brightness will eject electrons.
- A dim ultraviolet light is too weak to eject electrons -> If its frequency is above the threshold, even dim light ejects electrons immediately.
The physics behind it
To leave a metal, an electron must be given a minimum energy called the work function, W, measured in joules (J). In the photoelectric effect this energy comes from a single photon striking a single electron, so what matters is the energy carried by each individual photon, not the total energy of the beam.
Photon energy is set by frequency alone: E = hf, where h = 6.63 × 10⁻³⁴ J s. Red light has a relatively low frequency, so each red photon carries little energy.
If E is below W, the photon cannot release the electron, and photons do not combine their energy on one electron.
Worked line: suppose a metal has W = 3.6 × 10⁻¹⁹ J and red light has f = 4.3 × 10¹⁴ Hz. Each photon carries E = hf = 6.63 × 10⁻³⁴ J s × 4.3 × 10¹⁴ Hz = 2.9 × 10⁻¹⁹ J. Since 2.9 × 10⁻¹⁹ J is less than 3.6 × 10⁻¹⁹ J, no electron is emitted, however bright the light.
See it in daily life
A darkroom for developing photographic film is lit with a dim red safelight. Even though the darkroom worker turns it up to see clearly, the bright red light does not fog the film, because each red photon lacks the energy to trigger the light-sensitive reaction, while a single flash of white or blue light would ruin the film at once.
The same logic explains why gentle red heat lamps warm you without giving sunburn, whereas the far less intense ultraviolet in midday sun does cause burns. Sunburn depends on photon energy, set by frequency, not on how bright or warm the light feels.
It is also why a very bright torch shone on certain light sensors triggers nothing if its light is the wrong colour, but a much dimmer source of higher-frequency light sets the sensor off immediately. Piling on more low-frequency photons never adds up to one successful ejection.
How this comes up in SPM
In Paper 2 of SPM Physics (4531) this is a classic explain task: explain why increasing the brightness of light below the threshold frequency still produces no photoelectrons, and explain why frequency, not intensity, controls the effect. You should refer to the work function and the one-photon-one-electron rule.
You may also be asked to compare the effect of dim high-frequency light with bright low-frequency light, and to calculate a photon's energy from E = hf to show whether it exceeds the work function, keeping units in joules and hertz throughout. The command word relate links the threshold frequency f₀ = W/h to whether emission occurs.
Neighbouring content standards cover the photoelectric effect, the photon, and Einstein's photoelectric equation. Examiners look for the clear statement that a brighter beam simply sends more photons per second, each still too weak on its own.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)