The photon model of light is based on the idea that light comes in discrete “chunks” of energy. We say that the energy is quantized.

MODEL 38.1 Photon model of light

For use when quantum effects are significant.

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• Light consists of discrete, massless units called photons. A photon travels in vacuum at the speed of light.

• Each photon has energy

  $$E_{\text{photon}}=hf$$

  where $f$ is the frequency of the light and $h$ is Planck’s constant.

• The superposition of a sufficiently large number of photons has the characteristics of a classical light wave.

• Limitations: Use the photon model for light that is extremely weak or to analyze how light and matter interact at the atomic level. The classical wave and ray models are more appropriate for our everyday experience with light.

Workbook Exercises 16–18

Photons are sometimes visualized as wave packets, as illustrated in the photon model box. This electromagnetic wave has a wavelength and a frequency, yet it is also discrete and fairly localized. But this cannot be exactly what a photon is because a wave packet would take a finite amount of time to be emitted or absorbed. This is contrary to much evidence that the entire photon is emitted or absorbed in a single instant; there is no point in time at which the photon is “half absorbed.” The wave packet idea, although useful, is still too classical to represent a photon.

In fact, there simply is no “true” mental representation of a photon. Analogies such as raindrops or wave packets can be useful, but none is perfectly accurate.