I asked a lot of questions growing up. How do planes fly? What is sound? Why do hurricanes form? Why doesn't water fall out of my straw when I hold one end? Each time, the answer was differences in air pressure. Intuitively, this never made much sense, even as I went off to college. I couldn't see the differences in air pressure that were apparently all around, so I couldn't wrap my head around how they worked. This made me excited when I first came across schlieren photography, a photographic technique invented in the mid-1800's that enables you to visualize these density differences.
Schlieren photography is based on Snell's law, which describes how light bends when it moves between mediums of different densities. This concept is covered in depth in Project #6.
Illustration of how light changes direction when changing speeds. This relationship is given by Snell's law.
Given that pressure is a correlary of density, light will bend as it passes through different pressures. If you can somehow observe this bending, then you can see pressure changes. This is the core of classical schlieren imaging. Unfortunately, this technique requires a complex and precisely tuned setup of large parabolic mirrors.
There is a derivative of schlieren imaging called background-oriented schlieren. This technique is much more feasible to set up. In this project, I used background-oriented schlieren to visualize the airflow and density differences around a candle.