The photon energy is given by E = hν = hc/λ, where h = $6.63 \times 10$⁻³⁴ J·s, c = $3 \times 10$⁸ m/s, and 1 eV = $1.6 \times 10$⁻¹⁹ J. Using E(eV) = 1240/λ(nm), the energies for different parts are:

Radio waves (> 0.1 m): Energy < 10⁻⁵ eV (approximately 10⁻¹² to 10⁻⁵ eV)

Microwaves (0.1 m to 1 mm): Energy ~ 10⁻⁵ to 10⁻³ eV

Infrared (1 mm to 700 nm): Energy ~ 10⁻³ to 1.8 eV

Visible light (700 nm to 400 nm): Energy ~ 1.8 to 3.1 eV

Ultraviolet (400 nm to 1 nm): Energy ~ 3 to 10³ eV

X-rays (1 nm to 10⁻³ nm): Energy ~ 10³ to 10⁶ eV

Gamma rays (< 10⁻³ nm): Energy > 10⁶ eV

The different scales of photon energies are directly related to their sources. The wavelength of electromagnetic waves is correlated with the characteristic size of the system that radiates. Gamma rays originate from atomic nuclei (very small size, highest energy). X-rays are emitted from heavy atoms. Visible radiation is emitted by atoms when electrons move between energy levels. Radio waves are produced by accelerating electrons in circuits and antennas. Thus, smaller sources produce higher energy (shorter wavelength) radiation, while larger sources produce lower energy (longer wavelength) radiation. The classification of electromagnetic waves is based roughly on how the waves are produced and detected.