Intensity

Definition

Intensity is the power transferred per unit area in the direction of wave propagation. It is measured in watts per square meter ($$W/m^2$$).

5 Must Know Facts For Your Next Test

1. Intensity is proportional to the square of the amplitude of the electromagnetic wave.
2. 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.
3. 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.
4. Intensity decreases with distance from a point source according to the inverse square law: $$I \propto \frac{1}{r^2}$$.
5. 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.