Law of Reflection - Questions 5 Help

Getting your Trinity Audio player ready...

Hold down the T key for 3 seconds to activate the audio accessibility mode, at which point you can click the K key to pause and resume audio. Useful for the Check Your Understanding and See Answers.

A light ray reflects (bounces off) a plane mirror surface in a very predictable way. The angle at which it approaches the mirror is equal to the angle at which it reflects from the mirror. This is known as the law of reflection.

There are three similar versions of this question. Here is one of those versions:

Version 1:

The diagram shows an incident ray approaching a mirror surface. Tap on the diagram in order to identify the appropriate reflected ray.

About the Diagram
The diagram shows a plane mirror represented by the bold, thick angled line. The diagram also shows an incident ray - ray approaching the mirror - represented by the red arrow. The mirror and incident ray are placed on top of a protractor with angle divisions every 15 degrees. The protractor will be one of the most important tools in answering this question.

Applying the Law of Reflection
The law of reflection can be simply stated as ...

The angle at which the light ray approaches the mirror is equal to the angle at which it reflects off the mirror.

While technically, the angle of incidence or angle of approach is measured between the incident ray and the normal line (not the mirror), the measurement can just as easily be measured between the incident ray and the mirror surface. While this angle is not the angle of incidence, it does give a measure of the angle at which the light ray is approaching the mirror. The light ray must leave at this same angle. For example, if you measure the angle between the incident ray and the mirror to be 15°, then the angle between the reflected ray and the mirror must also be 15°. In this case, the angle of incidence and the angle of reflection (both 75°) would also be equal to each other.

To learn more about the law of reflection for plane mirrors, visit the following page at The Physics Classroom Tutorial:

The Law of Reflection