Candles on a cake, Schlieren image. The candle flames (white, lower centre) are heating air that is seen coiling (blue, red and yellow) as it rises. Schlieren photography is used to visualize density changes in transparent materials. This change in density causes light passing through a material to change direction (refraction). This property is measured by the refractive index of a material. A filter is then used to colour the light depending upon the amount it is deflected by the density change. Hot air is less dense than the surrounding air, which is also why it rises.