CAIE A-Level Physics 25.1.2 Inverse-Square Radiant Flux

Practise using F = L/(4πd²) to calculate flux, luminosity or distance for stars and galaxies, with appropriate units and inverse-square reasoning.

Exam points

use F = L/(4πd²) to calculate radiant flux intensity, luminosity or distance
rearrange the inverse-square relationship and use appropriate units

[Maximum number: 2]

The luminosity of the Sun is $3.83 \times 10^{26} \mathrm{~W}$ . The distance between the Earth and the Sun is $1.51 \times 10^{11} \mathrm{~m}$ .

Calculate the radiant flux intensity F of the Sun at the Earth. Give a unit with your answer.

unit

$\begin{aligned} F=\frac{L}{4 \pi d^{2}} =\frac{3.83 \times 10^{26}}{4 \times \pi \times 1.51 \times 10^{112}} \end{aligned}$

C1

$=1340 \mathrm{~W} \mathrm{~m}^{-2}$

A1

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25.1.2—The inverse square law for radiant flux intensity F question 1