A photon is a tiny packet of light energy that travels at the speed of light, c = 3.00 × 10⁸ m/s. Think of it as a “light bullet” that carries energy and momentum but has no mass.
The energy of a photon is directly proportional to its frequency:
\$E = hf\$
where \$h = 6.626 \times 10^{-34}\,\text{J·s}\$ is Planck’s constant and \$f\$ is the frequency in hertz (Hz).
Analogy: Imagine a musical note. The higher the note (frequency), the louder it sounds (energy). A photon’s “loudness” is its energy.
Even though photons have no mass, they still carry momentum:
\$p = \dfrac{E}{c} = \dfrac{hf}{c}\$
Example: A photon with energy 3.3 × 10⁻¹⁹ J has a momentum of about 1.1 × 10⁻²⁷ kg·m/s.
| Frequency (Hz) | Energy (J) |
|---|---|
| 5 × 10¹⁴ Hz (visible light) | ≈ 3.3 × 10⁻¹⁹ J |
| 1 × 10¹⁴ Hz (infrared) | ≈ 6.6 × 10⁻²⁰ J |
| 1 × 10¹⁵ Hz (ultraviolet) | ≈ 6.6 × 10⁻¹⁹ J |