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**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.

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