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The Angle of Incidence Equals the Angle of Reflection
A Physics Rule for Photographers
In physics, the law of reflection states that the angle of incidence is equal to the angle of reflection. This tenet is fundamental to the understanding of light and can be summarized thusly: if light strikes an object at angle A, it will be reflected in the opposite direction, also at angle A, similar to the way a ball bounces off a brick wall. In photography, the law of reflection is rarely discussed; one typically hears more about the Inverse Square Law or that white reflects and black absorbs. While these are indeed important aspects of light, the angle of incidence and the angle of reflection are two components of physics that, once understood, can help photographers improve their images they create in both artificial and natural light.
The highlights in the crystal flower are caused by the physics rule, "The angle of incidence is equal to the angle of reflection."
The easiest way to comprehend this concept is to go into a place that has hard, shiny floors and overhead lighting (grocery stores work great!). Look down while you walk and you’ll see hot spots of light on the floor move with you as you walk. These hot spots are the direct reflection of the overhead lighting, and they evidence the law of reflection. These equal angles of incidence and reflection can cause hot spots on your subject too. Understanding the law of reflection will help you avoid hot spots on your subjects, whether you are photographing models, cars, food, or landscapes. In fact, managing these equal angles of reflection in your photographs allows you to add or eliminate texture and color in your images.
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The law of reflection is also responsible for the red-eye effect that plagues ring flash users when shooting through the ring. Because the camera’s lens is at the same angle to the subject as the flash, the reflection of light against blood vessels in the retina at the rear of the eye produces red-eye. An easy way to eliminate red-eye is to brighten the room; this causes the subject’s pupils to contract, thus greatly reducing any reflection. Another method is to take a monolight with a 7-inch 20 degree grid and point it at your subject’s face with only the modeling lamp powered on (not the flash unit itself). Many flash units, including the Broncolor, Hensel, and Profoto brands have separate switches for the modeling lamp and electronic flashtube, allowing them to be powered separately.
The law of reflection is especially troublesome when glass or mirrors are present in the image. The equal angles of incidence and reflection cause hot spots in glass and mirrors when using a flash. The simple solution is to move the flash away from the camera so that the angles are not identical.
The bright, highlights on the candles are created when the angle of the light is equal to the angle of reflection into the camera lens.
In the studio, you can use the monolight red-eye reduction technique described above in a darkened room. This will allow you to show more of your subject’s iris and less of their dark pupils. The technique works well with light-colored eyes, especially green and blue. Don’t be alarmed by the appearance of harsh shadows on one side of the nose, as the power of the artificial flash will knock this out when it fires
By moving the camera and light source(s) independently, you can use the law of reflection in your favor, almost like an added layer of makeup to smooth your subject’s skin. As you walk around your subject, you will notice that hot and washed out spots will appear and disappear based on the angle of reflection. You may also notice that your model’s face appears smoother from one angle and rougher from another angle, as the valleys of the pores are filled in with shadows. Through positioning your camera and light sources independently, you can eliminate hot spots and create the appearance of a smoother skin texture.
Because the vast majority of what we see is reflected light (as opposed to incidental light), we as photographers live in an illuminated world. Without light, we would have no images to capture, and humans would see nothing but perpetual blackness. Understanding the law of reflection will allow you to outshine your competitors, as your photographs will take advantage of one of the fundamental laws of the universe and stand out from those created by your peers.