Inverse Square Law of Flux (Edexcel A Level Physics): Revision Note

Inverse Square Law of Flux

• The moment the light leaves the surface of the star, it begins to spread out uniformly through a spherical shell

  • Light sources which are further away appear fainter because the emitted light has been spread over a greater area

• The surface area of a sphere is equal to 4πr²

  • The radius r of this sphere is equal to the distance d between the star and the Earth

  • Therefore the radiation received at Earth has been spread over an area of 4πd²

• The inverse square law of flux can therefore be calculated using:

$F=\frac{L}{4\pi d^2}$

• Where:

  • F = radiant flux intensity, or observed intensity on Earth (W m^-2)

  • L = luminosity of the source (W)

  • d = distance between the star and the Earth (m)

• This equation assumes:

  • The power from the star radiates uniformly through space

  • No radiation is absorbed between the star and the Earth

• This equation tells us:

  • For a given star, the luminosity is constant

  • The radiant flux follows an inverse square law

  • The greater the radiant flux (larger F) measured, the closer the star is to the Earth (smaller d)

Inverse square law; when the light is twice as far away, it has spread over four times the area, hence the intensity is four times smaller