Photons
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 5 practice questions.
Light comes in packets called photons. Each photon carries energy E = hf = hc/λ: higher frequency (bluer) means more energy per photon. A brighter light just has more photons, not more energetic ones.
1. Energy of one photon
2. Using it
Across the spectrum, photon energy rises from radio waves (tiny) to visible light (about 2–3 eV) to X-rays and gamma rays (thousands to millions of eV).
That’s why ultraviolet can damage skin cells and visible light can’t: each UV photon carries enough energy to break chemical bonds.
Photons have no mass but they do carry momentum, p = h/λ. Light pushing on a surface is how solar sails work.
Power = (energy per photon) × (photons per second).
3. Example, then try it
Find the energy of a 500 nm (green) photon in eV and in joules.
E = hc/λ = 1240 ÷ 500 = 2.48 eV.
In joules: (2.48)(1.6 × 10⁻¹⁹) ≈ 4.0 × 10⁻¹⁹ J.
A 1.0 mW laser pointer emits 633 nm light. Find the energy of each photon, the number of photons per second, and each photon’s momentum.
Energy: 1240 ÷ 633 ≈ 1.96 eV = (1.96)(1.6 × 10⁻¹⁹) ≈ 3.1 × 10⁻¹⁹ J.
Photons per second: (1.0 × 10⁻³ J/s) ÷ (3.1 × 10⁻¹⁹ J) ≈ 3.2 × 10¹⁵.
Momentum: p = h/λ = (6.63 × 10⁻³⁴) ÷ (633 × 10⁻⁹) ≈ 1.0 × 10⁻²⁷ kg·m/s.
Answer: 3.1 × 10⁻¹⁹ J each, 3.2 × 10¹⁵ per second, 1.0 × 10⁻²⁷ kg·m/s.
4. Common mistakes
The frequency of a light source is doubled. What happens to the energy of each photon?
E = hf, so doubling f doubles each photon’s energy.
Ready to practice?
5 questions on Photons, each with a hint and an explanation for every wrong answer.