This article discusses angle of incidence. A fundamental concept that appears in many areas of photography. Getting a grasp of this concept will open up a whole new realm of possibilities for you as you move forward in your career or hobby.
An incidence is simply an occurrence, an action that takes place at a specific point. In our case this incidence is the point at which a particle of light strikes a surface. The law of physics states that whenever an object strikes another object there will be an equal but opposite reaction. Because we are talking about a particle of light, when that particle hits a surface it deflects off that surface in a very predictable way. Let's take our first example, a highly reflective surface.
Ir), travels from a light source towards a surface we wish to photograph. The angle at which it strikes that surface is called the angle of incidence (Ɵi). That angle is based on a defined angle that is perpendicular to the surface plane, otherwise referred to as the Normal (N). After that incidence ray strikes our surface it deflects, or reflects, off that surface and becomes our reflective ray (Rr). It leaves that surface at an opposite but equal angle called angle of reflectance (Ɵr).
To keep terminology easy to understand, that whole action, angle of incidence and angle of reflectance, is referred to simply as angle of incidence. But what if our surface wasn't smooth or flat?
The opposite is also true if the photographer is looking to keep a diffused light visible on the surface of the subject. By placing the light in an opposite but equal angle to the camera, that diffused light source will be reflected off the subject surface.
Another area where angle of incidence comes into play is when using reflectors either in a studio or outdoors in natural light. Product photographers will use small reflectors, called bounce cards, to bounce light to areas of shadow on their product. Natural light portrait photographers typically utilize large reflectors such as the 5-in-1 reflector to add light to open up shadowed areas or to add catch lights or hair light to their subject. Technically adept photographers will use reflectors in conjunction with strobes to achieve a dramatic look. Understanding angle of incidence makes using these tools easier, controllable and more predictable.
Getting back to that rough surface example. Even though a rough surface breaks up the angle of reflectance does not mean we can't use it to our advantage. In the above example of the product photographer, if that photographer requires a hard light then a small, highly reflective mirror will do the trick. For a softer, more diffused light the photographer can employ a white card that scatters the light for a softer look. Same goes for the portraitist using a reflector. A shiny silver reflector will have a harsher reflection with more contrast on the subject than a white reflector.
Knowing how each surface behaves under different conditions will enhance your photographic abilities as you put your knowledge of angle of incidence to use.
Go out and make pictures.