Intent

In this first of four activities that develop a technique for indirect measurement using mirrors and similar triangles, students explore what happens when light bounces off a mirror.

Mathematics

Students will discover through their experiments that the angle of approach of a light beam to a mirror equals its angle of departure. They will relate this principle to the use of a mirror to view the reflected image of an object.

Progression

Students conduct this in-class investigation in small groups, in which each student has a role, and share discoveries in a class discussion.

Approximate Time

30 minutes

Classroom Organization

Groups of 3

Materials

Flashlights (1 per group)

Mirrors (1 per group)

Protractors

Masking tape

Chart paper (optional)

Doing the Activity

You may need to partially darken the room for this activity. It will also help if groups tape over the flashlight lens to produce only a sliver of light. This will make the beam less dispersed and easier to see as a “line” bouncing off the mirror.

Because a flashlight beam is more easily seen against a plain background, you may want to suggest that groups tape chart paper to their desks. They can then trace the light’s path directly onto the paper.

The terms angle of incidence and angle of reflection refer to the angles between the light ray and a line perpendicular to the mirror, rather than between the light and the mirror itself. Because the angles between the ray of light and the mirror are more natural to work with, the student book speaks of the angle of approach and the angle of departure. Review the meanings of these terms with the class.

Figure 1

Students may have trouble knowing where to look in a mirror to find something that’s out of sight, and they may be confused at seeing directions reversed. You may want to give groups some introductory tasks to help students develop an intuitive sense of how mirrors work. For example, place a mirror on the ground and ask students to stand so that they can see the top of the board in the mirror. Or ask one student to hold a mirror so that another can see an object—like a window, door, or poster—reflected in it. Have them explain how they are deciding where to place the mirror and what happens as they move it.

As groups make their drawings, suggest that they record the mirror’s position as well as the path the light takes.

Discussing and Debriefing the Activity

When groups have had sufficient time to work with the flashlights and mirrors, bring the class together to discuss what they have learned. Theoretically, the angle of approach and the angle of departure should be equal. You might want to post the following observation:

Principle of light reflection: When light is reflected off a surface, the angle of approach is equal to the angle of departure.