Equation for the Intensity of Radiation (Edexcel International A Level Physics): Revision Note

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