Names:
In-Class Activity for Physics 7: Geometric Optics
1. Place a large concave mirror on a counter or table. Get as close to the mirror as you comfortably can, and look at your
reflection (everyone in the group should try this).
a) Is the image in the mirror upright or inverted? Is it larger than, smaller than, or the same size as your actual face?
Now, slowly back away from the mirror, keeping your reflection in view and watching for any changes in it. Again, each group
member should get a chance to try this.
b) At a certain point, you should notice a drastic change in your mirror image. Describe what this change is.
c) Based on your observations, give an estimate for the focal length of the mirror, in centimeters. Clearly explain your
reasoning. (Hint: how far were you from the mirror when you observed the drastic change in part (b)?)
2. On your laptop, open a web browser, go to the PhET site, and start the Geometric Optics simulation
(http://phet.colorado.edu/en/simulation/geometric-optics). A convex lens should already be in place in the simulation. In the
upper right-hand corner, check the box that says “Screen”; this makes the object a lamp, and gives you a black screen that can
be moved around. At the upper left, select “Many rays”; the simulation will display many white lines representing rays of light
emitted from the lamp.
At the lamp’s initial position, relatively far to the left of the lens, the rays that pass through the lens will be converged to a
point on the far side. If you place the screen at the point where the rays converge, the image on the screen will be in focus–it
will look like a small white dot, since our object here is basically a point source of light.
a) Try moving the lamp a little to the right, closer to the lens. (Try not to move it up or down yet.) What happens to the
point where the rays converge: does it move to the right, move to the left, or stay in the same place? Why does this happen?
The horizontal blue line through the center of the lens is called the optic axis or principal axis. Initially, the lamp should be
positioned with its bulb directly on the axis. Now, try moving the lamp a little above or below the axis, without changing the
horizontal distance between the lamp and the lens.
b) When you move the lamp up or down, what happens to the point where the rays focus? Does it move up or down? Does
it move to the left or right of the screen, or does it stay on the screen?
c) To continue this line of investigation: Select the box that says “2nd point”; this will give you a second lamp that produces
yellow rays. Place the second lamp directly above or below the first. Where does the light from the second lamp get focused?
Why do you think the term focal plane is used to describe the screen’s location in this case?
Try moving one lamp to the right, closer and closer to the lens, watching what happens to the rays.
d) Do the rays always converge (forming a real image), or are there some positions where the rays never converge? If the
rays don’t converge, what kind of image (if any) do we get? Note that the two yellow X’s represent the focal points of the lens;
how are those points relevant to this discussion?
e) Place the lamp so that its bulb is directly on the left focal point. Look at the rays that pass through the lens: what do
they look like as they emerge on the other side? Why?
The sliders at the top of the simulation allows you to adjust the refractive index of the lens, its radius of curvature, and its
diameter.
f) As you increase the index of refraction, what happens to the focal length of the lens? Why?
g) As you increase the radius of curvature, what happens to the focal length of the lens? Why?
h) As you increase the diameter, what happens to the focal length of the lens? Why?
Set the radius of curvature to 0.6 m, the refractive index to 1.5, and the lens diameter to 0.6 m. Place the lamp 120 cm from
the center of the lens–check the “ruler” box to measure the distance.
i) Use the ruler in the simulation to measure the focal length of the lens. Then check your result by calculating the focal
length using the lens-maker’s equation; show your work.
j) How far from the lens is the image (point where rays are focused)? Measure using the ruler, and also make a calculation
using the lens equation.
k) Move the lamp so that it is 90 cm from the lens. Where is the image now? (Measure with the ruler and check with a
calculation.)
Now, let’s look at images of an extended object. Uncheck the “Screen” box; the screen and lamp will be replaced by a pencil
and its image. Try moving the pencil closer to and farther from the lens– keep it to the left of the focal point for now.
l) As the pencil moves closer to the lens (outside the focal length), does the image size increase, decrease, or remain the
same?
m) Place the pencil 90 cm from the lens. Remember that the magnification of an image is defined as M = h0 /h, where h0
is defined to be negative if the image is inverted. With the pencil at 90 cm, what is the magnification of the image? (Make a
calculation; you may measure with the ruler to check your result.)
n) Is there a location for which the magnification will be equal to -1? If so, how far from the lens does the pencil need to be?
If not, explain why not.