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Activity 3: Plotting rays**

If you wanted to invent a new optical gadget, you would need to understand how light is reflected by various plane and curved mirrors.
Are there any patterns in the way plane mirrors and curved mirrors reflect light and form images?

What to use
Light box, concave mirror, convex mirror and plane mirror, white paper.
What to do
A    Plane mirror
  1. Set up the light box with a single slit to produce a single narrow ray of light.
    Light box with a single ray
    Plotting ray paths
  2. Shine this ray of light onto the plane mirror strip with the mirrors placed at different angles as shown in the diagrams.
    Plane mirrors at different angles
  3. Trace the mirror and the rays of light from the ray box to the mirror onto the paper.
  4. Show what happens to the light ray in each case after it hits the mirror.
  5. What happens if four parallel light rays are shone onto a plane mirror?
B    Concave mirror
  1. What happens if four parallel light rays are shone onto a concave mirror?
    Light box with four rays hitting a concave mirror
  2. Try it and sketch your results. Trace the mirror, the paths of the incident rays and the reflected rays.
C    Convex mirror
  1. Shine the four rays on the convex mirror and sketch your results.

Incident and reflected rays
Plotting the path of incident and reflected rays
The light rays that hit the surface of a mirror are called the incident rays.

The light rays that are reflected from the mirror are called the reflected rays.


  1. Try to work out a rule that will enable you to predict where a ray of light will go when it bounces off the surface of a mirror. You could start by looking at the angle between the mirror and the incident ray and the mirror and the reflected ray in each example in your plane mirror experiments.
  2. Which mirror (plane, concave or convex) caused the rays of light to be reflected so that they came together?
  3. Use your answer to the previous question to explain why one mirror shape could be used to focus a real image onto a screen and the other shape could not do this.
  4. Explain why the enlarged image of your nose seen in the concave mirror could not be focused onto a screen.
  • Find the name of the shape of a curved mirror that can give perfectly focused images.
  • What type of mirror is used in the following optical technologies, and in each case, is the image real or virtual?
    1. security mirror in a shop
    2. make-up mirror
    3. rear vision mirror
    4. astronomical telescope
    5. mirrors on corners of blind intersections.

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