Question 1

What makes a photon different from other particles?

Accepted Answer

A photon is unique because it has no rest mass and always travels at the speed of light in a vacuum. It's a fundamental packet of electromagnetic energy, unlike particles with mass such as electrons or protons.

Question 2

Does a photon's energy change if its speed changes?

Accepted Answer

A photon's speed in a vacuum is always the constant value 'c'. While light slows down in a medium, the photon's energy (determined by its frequency) remains constant. It's the wavelength that changes.

Question 3

Why are there so many formulas for photon momentum?

Accepted Answer

The different formulas (p=E/c, p=hf/c, p=h/λ) simply express the same physical quantity (momentum) using different known parameters of the photon (energy, frequency, or wavelength). They are all derived from each other and are useful in different problem contexts.

Question 4

If photons have no mass, how can they have momentum?

Accepted Answer

In classical physics, momentum is p=mv. However, in Einstein's theory of relativity, the full energy-momentum relation is E² = (pc)² + (m₀c²)². For a photon, the rest mass m₀ is zero, so the equation simplifies to E = pc, or p = E/c. This shows that a massless particle can indeed have momentum.

Energy and momentum of a photon

What you need to know

Energy and momentum of a photon