Spherical Mirrors






Bryan, Bill                  Joliet Catholic H.S.
12828 Blossom Drive          1-815-727-5311
Alsip, IL 60658
1-312-388-1361

Objectives

After this exercise the students should be able to:
  1. Construct ray diagrams of images in spherical mirrors; 
  2. Find images of objects in spherical mirrors; 
  3. Distinguish between convex and concave mirrors;
  4. Distinguish between converging light and diverging light. 

Materials Needed

  A. By student: calculator, protractor, shiny serving spoon, paper, pencil. 
  B. By teacher:  1. Spherical mirror (dinner plate size).
                  2. Box, painted black on the inside.
                  3. 2 light sockets, one painted black.
                  4. Light bulb.
  C. Optional equipment: Laser, mirage device.

Classroom Strategy

  A. Pre-class preparation:
     1. Place one light socket on one of the inside sides of the box.  Place the 
other light socket on the outside of the box, directly above the socket on the 
inside. Make sure the sockets are not on the bottom of the box. Now place the mirror 
at a distance which is twice the focal length of the mirror and insert the light bulb 
in the socket on the inside of the box. Adjust the mirror and the box so that you get 
a real image of the bulb above the socket on the outside of the box (the light needs 
to be on to get a clear image!). The apparatus is now ready for the demonstration. 

  B. Classroom Procedures:
     1. Set up the above apparatus prior to the class. Begin the class with the 
demonstration. Point out the object and its real image. Note that the image is 
inverted when compared to the object. This will be explained during the activity in 
class. If a laser is available, shine it over the top of the image and parallel to 
the principal axis. This beam will pass through the focus and over the top of the 
object. Another ray, directed over the image and through the focus will reflect off 
the mirror parallel to the principal axis and over the object. These rays are the 
same ones used in constructing a ray diagram, but now the students can see why we use 
these rays. 
     2. On the overhead projector, construct a ray diagram of what was just observed. 
If the laser was used, refer to the rays demonstrated. Have the students construct 
the diagram on paper as you do it on the overhead. 
     3. Have the students take out the serving spoon. Pass out paperclips and have 
them bend it so they have an object that will have a definite top and bottom. While 
viewing the concave side of the spoon, move the paperclip from far away from the 
spoon to close-up. Have the students note if the image is inverted or upright. Do the 
same for the convex side of the spoon. For the concave side, the image is inverted 
until you move inside the focus, where it will then be upright. The convex side never 
inverts the image. 
     4. On the diagram on the overhead, show how the parallel light in a concave 
mirror meets at the focus. Then show what happens to parallel light that reflects off 
the convex mirror. Distinguish between converging and diverging light. 
     5. Briefly show a convex diagram and where the image appears to be coming from. 
This will be explained in a future lesson. 
     6. Either put a problem on the overhead or have a handout with the problem on 
it, and have the students work it, using a ray diagram. A homework assignment will 
then follow. 
     7. The formula for finding image, object, and focus distances can also be 
presented, as well as height of the image and object. Or, these can be presented in a 
future lesson.     
     8. An image device, consisting of two concave mirrors face-to-face, can be 
purchased to project a real image of an object. This will produce the same type of 
image as the light bulb. The device can be displayed on the desk for students to 
ponder.