Angle of incidence

Whether you shoot portraits, landscapes or product photography, a basic understanding of the Angle of incidence and the law of reflection can help you figure out how to solve many different kinds of lighting problems in your photography.

Now, I’m not a physicist, I approach my understanding of topics here on my blog from a photographer’s point of view, and try to relate the base knowledge of the science to photography.  Making an image takes light, after all photography is defined as writing with light. Depending on the surface you are trying to photograph, that light can be a real curse.

The basics:

Light, whether it is from the sun or the strobe you are using to illuminate your subject is a waveform that moves in a straight line. When the light hits an object it is reflected, and the direction it goes is explained by the Angle of incidence, and Law of Reflection. The amount of light which is reflected also depends heavily on the material it is striking. This last point is a big one for any photographer.

Don’t Panic

I’m not going to bring in the formula for reflected light here, if you want to get that in-depth I invite you to google Angle of Incidence or Law of Reflection and you will find more than enough physics-based knowledge to keep you busy for a long while. The most important thing to know is: The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection. Sounds complicated, but what it means is that pointing a light at an object will result in the reflected light bouncing off at a very predictable angle. Understanding this angle can help you position yourself so that you don’t have lens flares in your image (unless you want the JJ Abrams look). Understanding angle of incidence in studio photography can also lead you to make more creative and interesting images.


Here we go, the above diagram illustrates our topic. For the purposes of photography, the ray labeled I is the incident ray and represents the sun or perhaps a studio strobe. Where it hits the mirror, is the normal line. The angle of the instance ray to the normal line is the Angle of Incidence. The ray labeled R is the reflected ray (the angle of reflection). There are two big things here, first -The angle of incidence is always equal to angle of reflection. This is how we can determine the reflection of a light off a surface in a very predictable way. The second big thing to understand is that as the angle of incidence increases, the angle of refraction also increases proportionally to the increase of incidence. For those of you who are like me and prefer a video example, here is something I found on YouTube that is quick and to the point.

Materials determine reflectivity

The example above is for light reflected off a mirror or other highly reflective surface. The thing you need to keep in mind as a photographer is the material the light is reflecting off alters the reflected light in a couple of ways: First, the amount of light reflected off is affected by the surface. A matte surface of say a painting or wall will diffuse and reduce the amount of reflected light, where jewelry or glass will not diffuse and will reflect directly. Second, the color of the material the light is reflecting off will absorb all but that color. A red shirt will reflect red light.

The unmodified light coming from your strobe, or the sun is white light. The wavelength of the light is measured in Kelvin temperature. It is odd to consider light being measured in temperature, but the temperature value determines the color of the light. Many strobes and the sun put off daylight temperature which is around 6500K. An incandescent lightbulb is around 3200K, and contains much more red light so the overall color cast is considered warmer. You can check out my three part series on light and light color here on the blog.

Experimentation with reflective objects can make a difference in your images whether you shoot portraits or products. A light source and a single reflector is all that is needed to make very dramatic portraits. When setting up a scene for a product, use different objects such as mirrors, or foam core boards to bounce light back into your scene and light it more effectively. I often use mirrors along with foam core to bounce light around and diffuse it in my scene. Using colored objects to bounce your light can cause unwanted color casts in your images so be wary of the objects you use to bounce light into your scene.

The takeaway here is that learning a bit about the light you are using can help you make more effective images. I strongly believe that anyone can set up two strobes at 45 degrees from your model and shoot flat frontal lighting. Why not take a single light and try to learn how to sculpt and reflect it to light your subject. Once you learn the basics of light sculpting, you can show or hide anything in your scene with ease.