To make or invent a new optical device, the dimensions and shapes of any mirrors used have to be mathematically correct, or the device will not work.
Law of reflection
When a ray of light is reflected, the angle of incidence equals the angle of reflection.
Can you use ray diagrams and calculations to solve some optical problems?
What to use
Flat mirrors, retort stands, concave mirrors with different curvature, a sheet of white paper or cardboard for a screen.
What to do
In this diagram of a ray of light striking a plane mirror the angle of incidence is 35°. What would be the angle of reflection?
Draw a diagram of a ray of light striking a mirror with an angle of incidence of 25°. Using this diagram or by using a calculation, determine the angle formed between the incident ray and the reflected ray.
Using two plane mirrors and two retort stands, design a periscope that could be used to look over a wall in front of you. What can be said about the usual arrangement and angles of the two mirrors in a periscope?
Is it possible to make a periscope with the two mirrors not parallel to each other? Try it. Draw a diagram to explain your answer.
Draw a diagram of the arrangement of the mirrors in a periscope that could be used to look over a wall that is behind you.
The focal length of a concave mirror is the distance between the mirror surface and a screen surface, on which you can see the focused image of an object that is at a great distance (eg on the horizon). Using this information, measure the focal length of two different concave mirrors. What is the relationship between the curvature of a concave mirror and its focal length?
When a concave mirror was used to focus an image of a very distant tree onto a screen, the distance between the screen and the mirror was measured. The same mirror is now used to focus an image of the window frame, which is much closer. Predict what must happen to the distance between the mirror and the screen for a clear image of the window frame to be formed. Test your answer.
Challenge
Denis and Mark stand directly in front of a plane mirror. Denis is 2 m in front of the mirror and Mark is 3.5 m from the mirror. Determine the distance between their images as seen in the plane mirror.
Plane mirror images
The image formed when you use a plane mirror is the same distance behind the mirror as the object is in front of the mirror.
The purpose of this activity is to allow students to use their mathematical skills to solve certain optical problems. Some problems require the student to use some simple apparatus to work out the solution.
The activity is written to produce higher level responses and is only suitable for those students who have the maths skills to cope with it.
