Photons
Photons
- Light is a wave — but it also arrives in countable lumps. This lesson prices one lump and counts how many a lamp emits each second.
Energy in a packet
- A is a packet (quantum) of energy of electromagnetic radiation. That two-part phrase is a two-marker, and the second mark dies without the word electromagnetic (9702/42/O/N/23 Q8(a)) (9702/41/O/N/25 Q8(a)).
Symbols
- = energy of one photon (J)
- = the Planck constant, 6.63 × 10⁻³⁴ (J s)
- = frequency of the radiation (Hz)
- = wavelength (using c = fλ) (m)
- Photon energy is proportional to frequency, so a gamma photon dwarfs a radio photon. Because it goes as , short-wavelength radiation carries the punchy photons.
- Photon energies are tiny, so we measure them in : (the energy an electron gains crossing 1 V). Convert eV→J by multiplying, J→eV by dividing — and papers demand the answer in whichever unit they name.
Worked example
Counting photons from a laser (2024 paper)
A 350 mW laser emits light of wavelength 640 nm. How many photons does it emit per second (9702/42/F/M/24 Q7(b)(i))?
- Energy per photon: .
- Photons per second = power / energy per photon: .
Answer
“Intensity” is really this number: brighter light means more photons per second, each still carrying the same .