# 8.1.3 Wave Intensity

### Wave Intensity

• Progressive waves transfer energy
• The amount of energy passing through a unit area per unit time is the intensity of the wave
• Therefore, the intensity is defined as power per unit area Intensity is equal to the power per unit area

• The area the wave passes through is perpendicular to the direction of its velocity
• The intensity of a progressive wave is also proportional to its amplitude squared and frequency squared Intensity is proportional to the amplitude2 and frequency2

• This means that if the frequency or the amplitude is doubled, the intensity increases by a factor of 4 (22)

#### Spherical waves

• A spherical wave is a wave from a point source which spreads out equally in all directions
• The area the wave passes through is the surface area of a sphere: 4πr2
• As the wave travels further from the source, the energy it carries passes through increasingly larger areas as shown in the diagram below: Intensity is proportional to the amplitude squared

• Assuming there’s no absorption of the wave energy, the intensity I decreases with increasing distance from the source
• Note the intensity is proportional to 1/r2
• This means when the source is twice as far away, the intensity is 4 times less
• The 1/r2 relationship is known in physics as the inverse square law

#### Exam Tip

The key concept with intensity is that it has an inverse square relationship with distance (not a linear one). This means the energy of a wave decreases very rapidly with increasing distance ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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