Yer a wizard, Harry! This hand looks like it is gathering the force necessary to hurl a fireball. Actually, it's just giving off regular body heat in a special type of photograph that manages to capture changes in air density.
The light travels either directly from or near to the object being photographed. It bounces off the curved mirror and converges back on a focal point.
The filtering device is often nothing more complicated than a blade or a piece of paper with an edge that cuts across the focal point of the light.
Under normal circumstances - when the air is a uniform density - the edge wouldn't really affect the picture, other than making it darker.
Heat and cold, however, expand and shrink air, making it less or more dense. Light bends as it moves through different densities of air.
If the light is bent enough, some of it will fall on the object that's blocking the focal point, creating dark spots on the picture.
That pattern of light and dark is then captured on film, as it is in the pictures of the hand and the turbulent air above a recently-extinguished candle.
