Candle, schlieren image. Air around a candle can be seen coiling and rising. Hot air is less dense than cold air, so when there is a relative temperature difference the hotter air rises in a process known as convection. Schlieren photography is a way of visualising density changes in transparent fluids. Light passing through a density change is refracted (bent). The imaging technique alters the brightness or colour of this refracted light to make such areas visible.