Wave intensity is the amount of energy a wave carries per unit area per unit time, usually measured perpendicular to the direction of wave propagation.

 What is Wave Intensity?

Definition:

Wave intensity is the amount of energy a wave carries per unit area per unit time, usually measured perpendicular to the direction of wave propagation.

Explanation:

The intensity of a wave tells us how strong or powerful the wave is at a given location. It depends on the amplitude of the wave, larger amplitudes mean higher energy and, therefore, greater intensity. Intensity also decreases as the wave spreads out over a larger area, following the inverse-square law.

Imagine:

Think of turning up the volume on a speaker. The sound waves don’t change speed, but their amplitude increases, and the sound becomes louder because the intensity has increased.

In simple terms:

Wave intensity measures how much “energy punch” a wave delivers over a certain area.

Formula:

I = P / A

Where:

 • I = intensity (W/m²)

 • P = power carried by the wave (W)

 • A = area through which the power is spread (m²)

Also, for sinusoidal waves:

I ∝ A² (intensity is proportional to amplitude squared).

Key Points:

 • Intensity shows how strong a wave is.

 • It decreases as distance increases (inverse-square law).

 • Proportional to the square of the amplitude.

 • Units: watts per square metre (W/m²).

 • Applies to sound, light, and all energy waves.

Examples:

 • A faint star vs. the sun: the sun has much greater intensity at Earth.

 • Louder music → higher sound wave intensity.

 • Laser beam vs. torchlight: laser has much higher intensity over a small area.

 • Earthquake P-waves and S-waves recorded with different intensities.

Applications / Relevance:

• 🔊 Acoustics – measuring loudness of sound

• 💡 Optics – brightness of light beams

• 📡 Telecommunications – signal strength

• 🌍 Seismology – measuring earthquake wave strength

• ⚡ Power transfer – evaluating energy carried by waves

Question:

Why does wave intensity decrease with distance?

Answer:

Because as the wave spreads out, its energy is distributed over a larger area, so the energy per unit area (intensity) becomes smaller.