Intensity of a wave is the rate of transfer of energy per unit area perpendicular to the direction of travel of the wave.

- Unit: $W m^{-2}$

$I = \frac{Energy}{Time \: \times \: Area}$

$I = \frac{Power}{Area}$

$I = \frac{Power}{4 \pi r^{2}}$, where $4 \pi r^{2}$ is the surface area of a sphere.

$I \: \propto \: \frac{1}{r^{2}}$

$I \: \propto \: (Amplitude)^{2}$

$\frac{I_{1}}{I_{2}} = \frac{r_{2}^{2}}{r_{1}^{2}}$

Loudness of sound and brightness of light depends on amplitude of wave.