Simple Schlieren Apparatus
A Schlieren apparatus  creates an image caused by inhomogeneities in density of a translucent medium. These in turn result in variations in refractive index, which cause light rays to be bent at different angles. This Demonstration models a focusing Schlieren apparatus in which you can vary the distance between the two grids and the density of the sample. You have the options of showing light rays moving through the apparatus and whether to show the typically invisible Schlieren sample. To the right of the apparatus is an image that lets you see the effect of changing the density of the sample and the grid separation on the final image. You can experiment with different densities and adjustments of the grid space for optimization of the projected image.
 Wikipedia. "Schlieren photography." (Jan 22, 2016) en.wikipedia.org/wiki/Schlieren_photography.