## R

Radiation, Absorption radiation is ENERGY transmitted by electromagnetic waves. Electromagnetic waves travel at the speed of light (when passing through a vacuum) and have a characteristic wavelength, X, which is inversely proportional to their frequency, v, by

X (m) = c (m s-1) / v (s-1), where c is the speed of light. Electromagnetic radiation is conceptualized in contemporary theory both as a wave and as a stream of particles called photons (this dual approach is referred to as wave-particle duality). The energy of any photon, E, of radiation is inversely proportional to the wavelength by

E = h v, where h is Planck's constant. This relationship allows us to order electromagnetic waves from high energy/ short wavelength (for example, x-rays), to low energy/ long wavelength (for example, radio waves). The resulting progression is referred to as the electromagnetic spectrum (Figure 1). The visible region of the electromagnetic spectrum is bound by infrared (IR) radiation on the lower energy side of the visible region (around

Figure 1: The electromagnetic spectrum

Figure 1: The electromagnetic spectrum

1 |im to 1 mm in wavelength), and by UV radiation (UV) on the higher energy side (from 400 nm to 1 nm). Microwave radiation is slightly lower in energy than IR, with a wavelength of around 1 cm.

All objects both emit and absorb radiation. Although all objects emit radiation at all wavelengths, the frequency of maximum emission, Xmax, is proportional to the temperature of the object by Wien's law

Xmax = a / T, where a is a constant equal to 2897 |im K. This implies that hotter objects emit higher energy radiation, as would be expected from everyday experiences. From Wien's law, the surface temperature of the sun can