• 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, 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 that?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