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
Set up the light box with a single slit to produce a single narrow ray of light.
Shine this ray of light onto the plane mirror strip with the mirrors placed at different angles as shown in the diagrams.
Trace the mirror and the rays of light from the ray box to the mirror onto the paper.
Show what happens to the light ray in each case after it hits the mirror.
What happens if four parallel light rays are shone onto a plane mirror?
B Concave mirror
What happens if four parallel light rays are shone onto a concave mirror?
Try it and sketch your results. Trace the mirror, the paths of the incident rays and the reflected rays.
C Convex mirror
Shine the four rays on the convex mirror and sketch your results.
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.
Discussion
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.
Which mirror (plane, concave or convex) caused the rays of light to be reflected so that they came together?
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.
Explain why the enlarged image of your nose seen in the concave mirror could not be focused onto a screen.
Challenge
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?
If students say they do not know what to look for to decide on a rule, lead them only as far as necessary by suggesting the first (or more) of the following steps. Once one group deduces the rule, get them to help others rather than doing it yourself.
Look at the angles at which the rays hit and bounce off the mirror.
Go back to each of your diagrams for the reflection of a light ray by a plane mirror and draw a line at right angles to the mirror at the point where the light ray was reflected. This line is called the normal.
Measure the angles of incidence and the angles of reflection in each of your diagrams.
How close in size are the angles of incidence and the angles of reflection for each diagram? How could you improve the accuracy of the angles you have drawn?
