• 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