Particle (quantum) theory. EM radiation is made of discrete packets — quanta (also called photons).

Q = hν = hc / λ

• Q — energy of a quantum, Joules.
• h — Planck’s constant, 6.626 × 10⁻³⁴ J·s.

• ν — frequency (Hz); λ — wavelength.

• Longer wavelength → lower energy per photon. That’s why long-wavelength sensors (thermal, microwave) must view larger ground areas — they need a bigger footprint to collect enough energy for a detectable signal.