If you'd like to know more about the essential role that light plays in photography, then you’ll love our course, A Photographer's Guide to Light. In this lesson, you will learn about direct reflections and see how they work in lighting and photography.
Direct reflection—also known as specular reflection—is the mirror-like reflection of a light from a surface in which the light from a single incoming direction is reflected in a single outgoing direction.
This direct reflection behaviour is described by the law of reflection: the angle of incidence equals the angle of reflection.
Here you can see a green laser, a mirror, and (printed on paper) a protractor. The light coming from the laser to the mirror is called the incident light, and the light that's leaving the mirror is called the reflected light.
If you imagine a line being drawn perpendicular to the surface of the mirror at the 90-degree mark, this is called the normal line. The normal line divides the angle between the incident light and the reflected light into two equal angles. The angle between the incident light and the normal is known as the angle of incidence. The angle between the reflected light and the normal is known as the angle of reflection.
No matter where the laser is moved, these two angles always equal each other.
In the images above, you can see a hard drive platter in a room with one light source. From the angle in the left image, the platter looks black. If the camera is moved in line with the reflection of the light source, though, it becomes much brighter. There’s a little light lost to absorption, but other than that, the reflection is almost as bright as the source itself.
This would seem at odds with the inverse square law because if the mirror is pulled back 30 more feet, as in the image above, the reflection of the light source would still be as bright as the light source itself, even though it's at a much greater distance.
The thing that actually changes, though, is the size of the reflection. Even though the light source appears to be as bright in the reflection, because the actual reflection is larger, it's reflecting more light.
If the light is moved to half the distance, it's going to reflect four times as much light—exactly what the inverse square law says that it will.
Objects With Lots of Direct Reflection
This leads us to an important concept when photographing objects that have a lot of direct reflection, where you need to give more thought to where you position the lights in objects to reflect back to the camera.
In other words, you don't think about how to light the object as much as you think about how to get the reflection of the light—which is pretty much the only thing that you're going to see—back to the camera.
This leads us to the idea of the family of angles.
The Family of Angles
The concept is fairly simple: if you’re shooting an object with a lot of direct reflection, there's a range of angles that produce this direct reflection. Anything outside this family of angles is not something that your camera can see. This is an important concept to understand because it’ll help you determine where to position your lights.
Because of the law of reflection, you can fairly easily determine where the family of angles is located with respect to the camera. If you’re shooting a mirror-like object and you want to see a direct reflection of your light source in the mirror, you need to use a light source large enough to fill the family of angles.
If you don't want to see a direct reflection of the light in the mirror, you need to position the camera and the light so that the light is not located in the family of angles.
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About the Authors
David Bode created the video course that includes this lesson. Dave is an expert on video and audio production, and he lives in the upstate New York area. He works as a camera operator, editor, inventor, motion graphics designer, recording engineer, and studio musician.