What is the Law of Reflection of Light? - Definition & Overview

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  • 0:00 Definition of Reflection Law
  • 1:40 More on Reflection
  • 2:15 Specular Reflection
  • 3:05 Diffuse Reflection
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson you will learn about the concept of reflection, including one of the simplest laws of physics, the law of reflection. You will learn about the main features of the law of reflection as well as some application and facts about reflection in general.

Definition of Reflection Law

Have you ever wondered why you are able to see your reflection in a mirror or why you see things reflected in the first place? An even more important question is why do we even see a table, or a chair or our phones sitting on the table? The answer to these questions happens to be one of the simplest laws in physics; it is called the law of reflection.

The Law of Reflection states that the angle of the incident light ray is equal to the angle of the reflected light ray. To understand what these angles and light rays stand for, consider the following diagram depicting the law of reflection. In this depiction, the blue band represents a mirror, which reflects rays of light.

Law of Reflection Illustrated
Reflection

The most important thing about the law of reflection is shown as a dashed line in the figure labeled the normal. The normal line is just a line drawn to the surface of the mirror that makes a 90 degree angle to the mirror. This line is used as a reference point for all of the angles in the law of reflection.

The incident ray is the beam of light that initially strikes the mirror and the reflected ray is the beam of light that bounces off the mirror after striking the mirror. The angle of incidence is the angle that the incident ray makes with the normal and the angle of reflection, or reflected angle, is the angle that the reflected ray makes with the normal. The equation for the law of reflection is given by the following formula:

The angle of incidence equals the ray of reflection.
Law

So this law states that any ray of light that strikes an object will reflect off the object such that the striking or incident angle is identical to the reflecting angle (as measured from the normal).

More on Reflection

The law of reflection tells us that light reflects from objects in a very predictable manner. So the question is, why do we see objects like a table or a chair? These objects do not produce their own light, so in order for us to see any object, light must strike the object and reflect from the object into our eyes. More specifically, in order for us to be able to see objects, the light reflecting off an object must make its way directly to our eyes. So how does the light get from the object to our eyes? It does so through one of the two types of reflection: specular and diffuse reflection.

Specular Reflection

Specular reflection is reflection off smooth surfaces. We can think of a beam of light as being composed of a bundle of many rays of light. When the beam hits a smooth surface like a mirror or a still pond, the rays collectively travel together with the same intensity and undisturbed.

You've seen specular reflection at work whenever you look at a peaceful lake, where the background is reflected across the surface of the water.

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