Law of Reflection
Light, Sound and Waves

Reflection of light in plane mirrors

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

The following activities can be used to introduce the law of reflection of light.

What to Prepare

  • small ball
  • projector and large plane mirror
  • narrow beam sources, plane mirrors and protractors
  • the interactive diagram, and the means to display it to the class (see below)

What Happens During this Activity

A short opening activity does not involve light but instead uses a small ball. Gather the class round one side of the room and take the ball. Roll it along the floor so that it hits the wall at about 45 degrees and bounces (reflects) away. Repeat this at a number of different angles, without comment, then ask the class what they notice about how the ball bounces off the wall.

Most pupils have an intuitive sense that the ball bounces away from the wall in a symmetrical way and that the angles are equal. If you open up the discussion here, you will soon find yourself deep in matters of snooker, basketball and the like.

Make the point that the aim of the lesson is not to think about balls bouncing off walls, but of light being reflected from mirrors.

Teacher: Where will the beam of light end up?

Set up a projector so that it casts a beam of light across the lab. You might place the projector so that it is somewhere in the body of the room, with the beam being directed towards the front. Switch all of the lights out and locate the beam with a white screen (a large piece of paper will do).

Now take a reasonably large plane mirror and hold it up towards the light beam. Before you place the mirror in the beam, ask the pupils to predict what they think will happen to the beam of light when it strikes the mirror.

You are likely to be told that the beam will be reflected. Ask the class to predict in which direction the light will be reflected. Who will it hit in the class? Will it be similar to the ball bouncing off the wall?

Now place the mirror in the beam and see where it lands. Repeat this, holding the mirror at different angles to the incident beam. Take care not to shine the beam into the face of any pupil.

It will soon become evident that the light is reflected from the mirror in the same way that the ball is reflected from the ball.

The angle going in is equal to the angle coming out.

In this final activity, checking out the law of reflection of light, direct the pupils to work in pairs with narrow beams and plane mirrors to check the emerging idea that when light is reflected the angle going in is equal to the angle coming out.

In practice the biggest problem here is created by the need to measure angles up to the normal line.

Resources

Download the software for this activity.

Law of Reflection
is expressed by the relation θ_i=θ_r
formalises Reflection
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