Understanding Angle of Incidence

One of the hardest concepts for beginners is understanding light and how it behaves. As I've said many times before, we can not see light, just the effect of light. Once you start thinking of light in terms of particles of energy then it starts becoming easier to understand. Specially since light behaves in very predictable ways.

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.

Light beams (red) travel from our light source (sun, strobe, fluorescent, etc.) and hits a reflective surface (mirror, metal, etc.). That light bounces off that surface and deflects (green) in an equal but opposite direction. In our example we see the light comes in at a 45º angle from the left and bounces off the reflective surface, exiting at a 45º angle to the right. Because our example surface is flat the light deflects all in the same direction which creates a very uniform and directed light. A surface like this is easy to work with since it's easy to recognize and predict. So what exactly is going on with all this physics stuff anyway?

I'm glad you asked. In the most simplest of terms a particle of light, or an incidence ray of light (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?

In this example we have a rough, irregular surface. Since we now know light behaves in a very predictable manner and we understand that it will deflect in an equal but opposite direction, we can see how each of the little angles of our surface deflects the particles in very predictable ways. Because all the angles are different light gets scattered or diffused. We may not have complete control over how the light interacts with this rough surface but because we understand the principle, we can allow for it and use it to our advantage. So why is this knowledge so important?

To begin with, whenever you are looking to photograph anything that has a high reflective value, such as metal, glass, water, plastics, and so on, knowing how light reflects off these surfaces makes it easier to avoid getting your light in the picture. For example, a product photographer taking a photo of a brand new shiny car. Or the landscape photographer taking photos of shiny glass skyscrapers. Or the portrait photographer taking photos at the beach or next to a lake.

Understanding that the angle at which the light hits your surface will be the same but opposite angle coming towards the camera, you can simply adjust the angle of the camera so it's not in direct angle to the light. Usually a small shift in height and or side to side can fix a potentially distracting problem.

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.