Schlieren Photography
Visualize the invisible. Schlieren imaging reveals density gradients in transparent media — convection plumes rising from heat sources, rendered visible through the optical interplay of a point light source, a knife edge, and refracted light. Click to place heat sources and watch thermal currents emerge.
About this lab
Schlieren photography is an optical technique invented by August Toepler in 1864 for visualizing density variations in transparent media. The method exploits the fact that light refracts when passing through regions of varying refractive index — which in gases is directly proportional to density. A point light source illuminates the test region, and a knife edge placed at the focal point of a receiving lens blocks light that has been deflected by density gradients. Regions where the gas is denser than average deflect light toward the knife edge (appearing dark), while less dense regions deflect light away from it (appearing bright).
The sensitivity of a schlieren system depends on the focal length of the optical elements and the fraction of the source image blocked by the knife edge. The angular orientation of the knife edge determines which component of the density gradient is visualized: a vertical knife edge reveals horizontal gradients, while a horizontal one shows vertical gradients. This makes schlieren imaging particularly powerful for studying convection, where heated air rises in plumes with sharp density boundaries against the cooler surrounding atmosphere.
In this simulation, a simplified fluid dynamics model tracks temperature and velocity on a grid. Heat sources warm the fluid, creating buoyant plumes through the Boussinesq approximation — density decreases linearly with temperature, driving upward flow. The schlieren visualization is computed by taking the directional derivative of the density field along the knife-edge normal, mimicking the real optical process where light deflection is proportional to the integral of the refractive index gradient along the optical path.