Edexcel A Level Physics:复习笔记5.12 Equation for the Intensity of Radiation

• 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