Intensity of a Wave (OCR A Level Physics): Revision Note

Intensity of a Progressive Wave

• 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

• The unit of intensity is Watts per metre squared (W m^-2)

• 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

• 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