phy1020.DVI

• Whether the image isrealorvirtual. (In a real image, light is present at the image location, and the
  image can be projected onto a screen. In a virtual image, there is no light present; a virtual image
  cannot be projected onto a screen.)


• Whether the image isupright(rightside-up) orinverted(upside-down).

There are two methods that can be used to solve this type of problem:

• Theray diagram methodis a graphical method. It gives a good intuitive picture of what’s going on, but
  it can be a bit time-consuming, and is not particularly accurate.


• Thealgebraic methoduses only algebra. It doesn’t give a good picture of what’s happening, but it’s
  faster and more accurate. However, the algebraic method requires that you are very careful with the
  equations, particularly with regard to getting the signs correct.

We’ll cover both methods here.

45.1 Ray Diagrams

Aray diagramis used to locate the image produced by a mirror. To create such a diagram we draw the mirror,
its axis, the object, and three light rays, as shown in Fig. 45.3. We also need to locate the focusFand center
of curvatureCalong the mirror’s axis. The three rays we draw are:

1. In parallel to axis, out through the focus.


2. In through the focus, out parallel to the axis.


3. In through the center of curvature, and back out through the center of curvature.

(Only two rays are really needed; the third acts as a check.) The image will be located at the point where the
three outgoing rays meet, as shown in the figure. If the outgoing rays donotmeet (i.e. they diverge), then
trace the outgoing rays back behind the mirror; in this case you will have a virtual image.

3

OBJECT

IMAGE

1

2

3

F

C

Figure 45.3: Ray diagram for a converging (concave) mirror.