Photon
Quantum Physics - OCR A-Level Physics
Key Definition
Photon
A quantum (discrete packet) of electromagnetic radiation. A photon has zero rest mass and travels at the speed of light c.
A quantum (discrete packet) of electromagnetic radiation. A photon has zero rest mass and travels at the speed of light c.
$$\begin{aligned}
E &= hf \\
&= \frac{hc}{\lambda}
\end{aligned}$$
- Electromagnetic radiation is emitted and absorbed in discrete packets called photonsDiscrete packets (quanta) of electromagnetic energy. Energy E = hf, where h is Planck's constant., not as a continuous wave.
- The energyThe capacity to do work. Measured in joules (J). of a single photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. depends only on its frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz).: higher frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). means higher energyThe capacity to do work. Measured in joules (J)..
- Gamma photons are the most energetic; radio photons are the least energetic.
- The intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. of a beam of light depends on the number of photons per second and the energyThe capacity to do work. Measured in joules (J). of each photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency..
$$E = hf$$
$$E = \frac{hc}{\lambda}$$
Worked Example [2 marks]
Calculate the energy of a photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. of green light with wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). 530 nm.
Show Solution
1
Convert
$\lambda = 530 \times 10⁻⁹\;\text{m}. Use E = hc/\lambda = (6.63 \times 10⁻^{3}⁴ \times 3.00 \times 10⁸) / (530 \times 10⁻⁹)$
[1]2
$E = 3.75 \times 10⁻¹⁹ J$
[1]Answer
$E = 3.75 \times 10⁻¹⁹ J (or 2.35 eV)$
Common Mistake
MEDIUM
Students often: Using wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). in nanometres directly in $E = hc/\lambda$ without converting to metres.
Instead: Convert wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). to metres first. For example, 530 nm = 530 × 10⁻⁹ m = 5.30 × 10⁻⁷ m.
Instead: Convert wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). to metres first. For example, 530 nm = 530 × 10⁻⁹ m = 5.30 × 10⁻⁷ m.