Intensity is proportional to the square of the amplitude of the electromagnetic wave.
In a vacuum, intensity can be calculated using the formula $$I = \frac{P}{A}$$, where $$I$$ is intensity, $$P$$ is power, and $$A$$ is area.
The average intensity of an electromagnetic wave can also be expressed as $$\langle I \rangle = \frac{1}{2} c \epsilon_0 E^2_{max}$$, where $$c$$ is the speed of light in a vacuum, $$\epsilon_0$$ is the permittivity of free space, and $$E_{max}$$ is the maximum electric field strength.
Intensity decreases with distance from a point source according to the inverse square law: $$I \propto \frac{1}{r^2}$$.
For a monochromatic plane wave traveling through free space, intensity remains constant along any plane perpendicular to the direction of propagation.
Review Questions
How does intensity relate to the amplitude of an electromagnetic wave?
What formula would you use to calculate intensity given power and area?
Why does intensity decrease with increasing distance from a point source?
Related terms
Power: The rate at which energy is transferred or converted. It is measured in watts (W).
Amplitude: The maximum extent of a vibration or displacement of a sinusoidal oscillation measured from its equilibrium position.
Inverse Square Law: A principle stating that a specified physical quantity or strength decreases proportionally to the square of the distance from the source.