1. The angle of incidence equals the angle of reflection.
Seems like an obscure little law of physics to have much impact on photography, but believe me it has a huge impact. Take an example like the one above, to make sure the prop blur shows up well you want your position to be place such relative to the aircraft that the sun will reflect off the propeller into the lens of your camera, thus making the propeller show up better than it otherwise would. Also in product photography, this is one of the most important considerations. Many products are very reflective and placement, shape, and intensity of reflections can make or brake this type of photo.
2. Two points denote a line and three points denote a plane.
This bit of geometry determines how camera stabilizing devices work. A tripod is the most stable device because it has three points of support, making the camera stable on all axes. Systems with two points of support will prevent the camera from moving on one axis, but not the others.
3. The intensity of light is inversely proportional to the square of the distance from the source.
This law, known as the inverse square law, determines how light will “fall off” on your subjects. If your light source is close to your subject’s face, then there will be a significant difference between the distance between the light and her face, and the light and her shoulders, chest, abdomen, etc. On the other hand, if the light source is very far away, such as the sun, then the difference in distance will be negligible and there will be no appreciable fall off in distance. This law comes in very hand for managing the amount of drama in your images.