A photon is the smallest possible packet of electromagnetic radiation, including visible light, radio waves, ultraviolet light, and X-rays. It has no rest mass, but it carries energy and momentum, so it can interact with matter in measurable ways. Photons matter because they connect waves, particles, energy transfer, and quantum physics in one idea.
They explain why light can behave like a smooth wave in some experiments and like individual particles in others.
The energy of a photon depends only on its frequency, so higher-frequency light has more energetic photons. Even though a photon has zero rest mass, it still has momentum because it always travels at the speed of light in vacuum. The photoelectric effect shows that light transfers energy in packets, while the Compton effect shows that photons can transfer momentum during collisions with electrons.
Together, these experiments give strong evidence that light is quantized.
Key Facts
- Photon energy: E = hf, where h = 6.626 x 10^-34 J s.
- Frequency and wavelength: c = fλ, where c = 3.00 x 10^8 m/s in vacuum.
- Photon momentum: p = E/c = h/λ.
- A photon has zero rest mass but nonzero energy and momentum.
- In the photoelectric effect, electrons are emitted only if hf is greater than or equal to the metal work function φ.
- Compton scattering shows photon momentum because scattered X-rays change wavelength after colliding with electrons.
Vocabulary
- Photon
- A photon is a quantum, or smallest packet, of electromagnetic radiation.
- Frequency
- Frequency is the number of wave cycles passing a point each second, measured in hertz.
- Wavelength
- Wavelength is the distance between matching points on a wave, such as crest to crest.
- Work function
- The work function is the minimum energy needed to remove an electron from a material.
- Compton effect
- The Compton effect is the scattering of photons by electrons that changes the photons' wavelength and direction.
Common Mistakes to Avoid
- Using E = hcλ instead of E = hc/λ is wrong because photon energy increases when wavelength decreases.
- Saying photons have no momentum because they have no mass is wrong because photons have zero rest mass but still have momentum p = E/c.
- Thinking brighter light always ejects electrons in the photoelectric effect is wrong because the photon frequency must be high enough to overcome the work function.
- Treating a photon as only a tiny ball is wrong because photons show both particle-like energy transfer and wave-like interference behavior.
Practice Questions
- 1 A green photon has frequency 5.60 x 10^14 Hz. Calculate its energy in joules using E = hf.
- 2 A photon has wavelength 500 nm. Calculate its momentum using p = h/λ.
- 3 In a photoelectric experiment, red light is very bright but emits no electrons, while dim ultraviolet light does emit electrons. Explain what this shows about photon energy and frequency.