How To Do Schlieren Photography
So, how do you practically take these sorts of photographs? Now, the set up is a bit more complicated than what you're used to, but it's well worth it:
You need a parabolic mirror that is large enough to reflect your subject matter back into your camera. Most people don't have human-sized parabolic mirrors in their home, so you might want to ask around your local physics departments if they have a set up you could use. For small demonstrations, many cosmetics mirror can do our purposes. Telescope mirrors are preferable, however, and can be purchased for around 50$ at a decent size online. Spherical is preferable, but those are hard to come by at friendly prices.
Now, every parabolic mirror has a “focal distance", at which the light from it focuses: you'll want your camera twice that away from the mirror. This is exactly the same as that magic distance between a magnifying glass and whatever you're trying to magnify where everything snaps into focus.
Your next piece of equipment is a point light source. This is easy enough to make: take some sort of bulb, LED or otherwise, poke a hole in a piece of tinfoil or some other reflective material that won't light on fire, and cover it up! The brighter the light, the better: after all, all of your light will be coming out from this single point. Make sure you brush up on your usual low-lighting techniques, such knowing your aperture and ISO settings, though be mindful of your exposure as you'll need it to be short.
You have two options from here: you can use either the edge of a knife to force the light to diffract, or a special color filter. It's hard to get the knife edge set up to work, but basically the end of the knife edge needs to be at the focal point of the camera. As far as the special colour filter method is concerned, you need to have this pattern printed on a transparency and placed in front of the camera. Try both methods!
Now, it'll take a fair amount of adjusting to get the set up just right, so be patient. As far as subjects are concerned, nothing fancy is really necessary—a lit candle, a friend with a cold, anything that creates temperature or pressure differences can be used.