Astronomers can measure the distances of far-away galaxies by using standard candles. If we know the strength of a source of light, then we can calculate the distance to it based on the intensity of light we detect here. The intensity of light emitted from a point source decreases according to the inverse square law, based purely on spherical geometry. A standard candle is one such cosmic body, whose absolute luminosity is a known quantity. Type-Ia supernovae are particularly useful standard candles since they are believed to all have essentially the same absolute luminosity. In this demonstration, you can measure how the observed luminosity of a standard candle will decrease according to an inverse square relationship. The number of photons emitted from the source are divided among an increasingly large area (the variable-sized surface) – an area that increases with distance squared – and so measuring the fraction of the absolute intensity ($I$) over a standard area of detection (the fixed-size screen) will directly inform astronomers how far away this cosmic object is ($r$).

Use the slider to change the distance of the standard candle. Drag the demonstration to rotate the view. Observe that the photons becomes increasingly spread out as the distance from the standard candle increases, and hence the observed intensity of the light source decreases.