A photon is a tiny, indivisible packet of light. Think of it as a “light‑ball” that carries energy and can also push on objects, just like a tiny bowling ball can knock over pins. Photons have no mass, but they do have momentum, which is why they can exert pressure on surfaces (the radiation pressure we see in solar sails).
The energy of a photon depends on its frequency (or wavelength). The two most common ways to write it are:
Where:
Even though photons have no mass, they carry momentum given by:
\$p = \dfrac{h}{\lambda}\$
This tiny push is enough to move a solar sail across space or to create a measurable force on a delicate instrument in a laboratory.
When light shines on a metal surface, it can knock electrons out of the metal. This happens only if the photon’s energy is large enough to overcome the metal’s work function (the energy needed to free an electron). The relationship is:
\$E{\text{photon}} = \phi + KE{\text{max}}\$
Where:
If \$E_{\text{photon}} < \phi\$, no electrons are emitted, no matter how bright the light is. This explains why a very bright but low‑frequency (red) light can fail to produce photoelectrons, whereas a dim but high‑frequency (ultraviolet) light can.
| Quantity | Formula | Units |
|---|---|---|
| Energy of a photon | \$E = h\nu = \dfrac{hc}{\lambda}\$ | Joules (J) |
| Momentum of a photon | \$p = \dfrac{h}{\lambda}\$ | kg·m/s |
| Photoelectric condition | \$E_{\text{photon}} \ge \phi\$ | J |
| Maximum kinetic energy of photoelectron | \$KE{\text{max}} = E{\text{photon}} - \phi\$ | J |