Name: ________________________________
Partner: ________________________________
Date: ________
Period: ________
JUST A BORING PLANE MIRROR LAB
Suppose you look in a mirror and see your reflection. This reflection is called the image. You are
considered the object. In physics, we treat an image almost as if it really exists. The image can be bigger
or smaller than the object, and it even has its own position "behind" or "in front" of the mirror.
Part 1 - Ray Tracing
You have cardboard, a mirror, and three pins. Place a clean sheet of paper on the cardboard, and position
the mirror in the middle of the paper.
Mark the position of the mirror by tracing a line along the back of the mirror. This will help you place
the mirror in the exact same spot if it gets moved.
Place a pin in the cardboard close to the far edge, so that you can see
its reflection in the mirror. The diagram to the right is a good
example. Mark this pin as the Object Pin. Do NOT move the
Object Pin at all during this part of the lab.
Pin
Mirror
Place your eye well off to the side of the Object Pin (again, reference
the diagram). Close the other eye. Take a second pin and put it in
front of your eye so that it blocks the "Image of the Object Pin." Stick the pin into the paper to mark the
spot.
NOTE: Ignore the reflection of the second pin. You want the pin to block the image of the object pin.
You do not want to line up the reflections.
Place the third pin so that it also lines up with everything. It will either be between your eye and the
second pin, or between the second pin and the mirror, depending on where you have room for it.
You should now have the "Image of the Object Pin" and the two other pins all in a row. Later, the
locations of the two pins will be used to make a straight line which points at the Image.
Remove the last two pins (not the object pin), and label the holes they made in the paper (label them
both A). Move your eye to a new location that’s about 2 cm to the right or left of where it was before.
Repeat the above procedure to place the two pins in line with the image. Label these pin positions B.
Move your eye to a third position, and move the two pins so that they are once again aligned with the
object pin’s image. Label these positions C.
There should now be three sets of data.
Remove the mirror. You will now use a technique called Ray Tracing to find the position of the image.
Using the first set of data points, draw a line through the two points to make a line. Extend this line to
both ends of the paper. Repeat this for each of the 3 sets of points.
Where the three lines intersect is called the Image Position — it is the point your brain thinks the light
is originating from.
Draw a dark line connecting the Image Position with the Object Position. Then measure and record the
following below and on your diagram:
Object Distance: __________ (Distance from Object Position to the mirror)
Image Distance: __________ (Distance from the Image Position to the mirror)
Q1) The object distance is significantly larger than / significantly smaller than / about the same as the
image distance.
Part 2 – Angle of Reflection
Imagine the path a light ray would travel, starting from the object, going toward the mirror, bouncing
off, and finally coming to your eye. Your brain thinks that the light is coming from the image
behind the mirror! However, we know the light really came from the object.
Remove all the pins (yes, NOW you can move the object pin) and flip over the paper.
As before, place the mirror at the center of the page, mark the mirror position, and place the object pin
near a corner of the paper so that you can see its reflection in the mirror.
Repeat the procedures from the front page of this lab, getting 3 sets of data points that all line up with
the image of the object pin. But once you’ve got the data points, don’t draw the lines quite the same
way.
This time you’re going to draw lines that show the path that
light actually travels, as shown in the diagram to the right.
Draw a line through a pair of data points until it touches the
line representing the mirror. Then draw a line from this
contact point directly to the object pin.
Measure the angle between the mirror and the line going to the
object; label this ‘I’ (I).
Measure the angle between the mirror and the line going
through the pair of pins; label this ‘R’ (R).
R
I
object
Draw these same lines for the other 2 data sets, and measure their angles as well. Label these angles on
your diagram. Place all 3 sets of data into a table below.
Q2)
In general, I is significantly larger than / significantly smaller than / about the same as R.
Q3)
Write a general rule relating Object Distance and Image Distance when looking at a plane mirror
(a plane mirror is another name for a regular, flat mirror).
Q4)
Write a general rule relating the angle that light approaches the mirror and the angle that light
reflects off of the mirror.
Q5)
Using all of the data that you’ve collected, summarize in a sentence, where is the image of the
object located when looking at a plane mirror?