Name (printed) _______________________________
LAB
W
REFLECTION FROM A PLANE MIRROR
E LOVE TO look at plane
mirrors. We look at them as we
enter and leave bathrooms. At
the gym we work out in front of
them. We can’t help taking a
quick look at the mirrored walls
of elevators. But let’s be honest. It’s not the mirror
we’re looking at. No, we’re really looking at …
ourselves! (Well technically, we’re looking at images
of ourselves.) The storefront windows of businesses
act well enough as plane mirrors that we use them to
check ourselves out while we pretend that we’re
looking through them at merchandise. We love to
look at ourselves and because of this, it’s hard to find
places where there are no mirrors. To make sure we
can always get an “image fix” we carry
them in our purses and backpacks. I’m
sure that our prehistoric ancestors were
probably just as fixated by the images
they saw of themselves in flat pools of
water.
We use plane mirrors more often
than curved mirrors because plane
mirrors produce images that are the
best representation of ourselves. The
image produced by a plane mirror is
identical in size and orientation to its
object. On the other hand, curved
mirrors (concave and convex) produce
images that are sometimes larger,
sometimes smaller, and sometimes
upside down. What is it about plane
mirrors that causes them to produce so
identical an image? Answering that is
one of the goals for this lab. Starting
with plane mirrors is a great way to
start thinking about optics (and about
physics in general). It’s easy to make
some simple measurements that lead
directly to some general rules (aka
“laws of nature”).
It’s true that plane mirrors produce simpler
images than curved mirrors, but they certainly aren’t
trivial. Ask a group of people who have never studied
optics where they think the image produced by a
plane mirror is located and you’ll get three responses.
Some will say, “On the mirror surface, of course.”
Others will say, “In front of the mirror’s surface, of
course.” Still others will say, “Behind the mirror’s
surface, of course.” People tend to be emphatic about
what they know to be true. But all three locations
can’t be right. This lab will lead you to the discovery
of this image position. More important will be the
discovery of two laws of reflection, one of which is a
universal rule that applies to all reflections, not just
those from flat surfaces.
The image
produced by a
plane mirror
gives the best
representation
of an object.
The image size
and orientation
are identical to
the object. (Can
you tell that
there is only
half a house of
cards?) Photo
by Haydn Wall,
Class of 2014.
In the space below, write down where you think the image produced by a plane mirror is located.
Include some explanation or evidence for your answer (even if it’s just your intuition).
1
PURPOSE
To become acquainted with the qualitative and quantitative relationships which
exist between the object and its corresponding image in a plane mirror.
Specifically, you will discover:
- the position of the image with respect to the object.
- the relationship between the angle of incidence and the angle of reflection.
PROCEDURE:
1.
PART A – SINGLE MIRROR REFLECTION
Place the sheet labeled “Part A” of the lab on the piece of cardboard.
normal
line
incident reflected
angle
angle
incident
reflected
qi qr
ray
ray
reflective surface
2.
Place the mirror with the attached wooden block on the line marked “mirror”, so that the reflective side is flush with the
paper and faces the point marked “Object.” These are rear-surfaced mirrors so the backside of the mirror should be on
the line marked “mirror.”
3.
Place a florist pin (it has a little ball on top) on the point marked “Object.”
4.
Place the two regular pins on the points “1” and “1a.” Make sure all pins are vertical.
5.
With your eye at table level look at the two regular pins at points “1” and “1a” and note how these pins line up with the
image of the florist pin. Be absolutely sure each person sees this alignment before you go on. It’s a lot like looking (from
end view) at three people all standing in a line, and only seeing the first person because the other two are exactly behind
that first person.
6.
Look into the mirror from a viewpoint well to the right of the object. Place the two regular pins so that they line up with
the image of the florist pin as before. Label these points “2” and “2a.”
7.
Repeat this for a third angle, on the right or left side. Label these points “3” and “3a.”
8.
For each of the three sets of points, draw a dashed line representing the path that light appears to be traveling from the
image to the eye. The first one is done for you already.
9.
Use these lines to identify the image position, and label that point “image.”
10. Place the second florist pin at this image position. Notice how this pin is located exactly where the image of the first pin
appears. Move your head from left to right and notice that the image always appears to be at the same position. You are
using parallax to find the image position. Be sure you understand parallax, as you will need to use it in part B.
11. For each of the three pairs of points (1/1a, 2/2a, 3/3a), draw a solid line representing the path that light actually takes as
it goes from the object to the mirror to the eye.
12. Draw arrows on all light rays indicating the direction of actual and apparent light. See diagrams.
13. Draw normal lines (right angle) to the mirror at each point where the solid lines touch the mirror.
14. Measure and record the angles of incidence and the angles of reflection.
15. Now transfer the information from one module to another. This is simple. You only
need to transfer a few critical points to properly construct the ray diagrams. Just put
the completed single lab page right on top of the same blank page of the other module,
making sure the corners are exactly in alignment, and then push the pins through. The
new lab page has been marked and the diagrams are now easy to construct.
2
Ray diagrams lines
mirror
actual ray
apparent ray
normal line
DATA & ANALYSIS
1.
2.
3.
4.
The apparent path length of light is the path that light
seems to travel according to our eye. In centimeters,
measure and record the apparent path length of light
from the image to the eye.
Sighting
Incident
Angle
Reflected
Angle
Apparent
path
length
Actual
path
length
1
The actual path length of light is the path that light
really travels from an object to our eye. In
centimeters, measure and record the actual path
length of light from the object to the mirror to the
eye.
2
3
The object distance is measured from the object
perpendicularly to the mirror. In centimeters,
measure and record the object distance.
The image distance is measured from the image
perpendicularly to the mirror. In centimeters,
measure and record the image distance.
Object Distance:
_______________
Image Distance:
_______________
Mirror
1
1a
Eye
Object
Part A
GET CHECKED BEFORE MOVING ON
3
4
PROCEDURE:
1.
PART B – TWO MIRROR REFLECTIONS
Set up the two mirrors precisely on the mirror lines shown below.
2.
Place one florist pin at the point marked “Object.” Be sure it is aligned exactly vertically.
3.
Now place the other florist pin at point 1. This is a “locater pin” that marks the location of one of the images
formed by the object. Using your knowledge of parallax verify that this is the position of the image. (Note: this
is a different process than in Part A.)
4.
Look carefully into the mirrors - you should see two additional images (one in the other mirror and one in the
“corner” where the two mirrors meet). Use parallax to find these other two images and then label all three image
points with an “I.” This takes time - be patient!
5.
Use a dashed line to draw the apparent path the light takes from each of the images to the computer generated
eye. (Note: when locating the images, you don’t have to put your eye in the position of this computer-generated
eye. It is only provided so that when drawings are made, they will all be similar, throughout the class.)
6.
Now use your knowledge of the relationship between incident and reflected angles and your knowledge of the
relationship between the apparent and actual path lengths to draw solid lines for the actual path light takes in
going from the object to the eye. The “corner” image is tricky. To do this correctly, you must not violate the
relationship that you discovered in Part A between incident and reflected angles.
7.
Draw arrows on all light rays indicating the direction of actual and apparent light.
Part B
Mirror
Object
Image #1
Mirror
Eye
5
DATA & ANALYSIS
1.
In centimeters, measure and record the apparent path length
of light for the “corner” image, that is the distance from the
“corner” image to the eye.
2.
In centimeters, measure and record the actual path length of
light for the “corner” image, that is the distance from the
object to the mirrors to the eye.
3.
Calculate the percent difference between apparent path length and actual path length.
Percent Diff. =
Exp1 - Exp 2
( Exp1 + Exp 2 ) / 2
apparent path length
actual path length
´ 100%
QUESTIONS
1.
Summarize your observations about the behavior of light as it strikes a plane mirror (this should include
statements both about incident vs. reflected angles as well as apparent vs. actual path lengths.
2.
Explain (and draw a picture) how plane mirrors put on walls can make rooms look larger.
3.
The astronauts put a corner-reflecting mirror on the moon. This is a
combination of mirrors that are joined at 90° angles – like the photo to the
right (taken by David Angelo and Michael Barbaro, Class of 2004). The
mirrors on the moon are designed to reflect laser beams sent from the Earth
directly back along a line parallel to the path they came on. The perpendicular
lines below represent two plane mirrors at right angles. Draw normals (I drew
the first for you) in order to measure and draw angles. Complete the drawing
and then bring in another incident ray in a direction that is different
from the first, again completing the path, in order to illustrate this property
of corner reflectors.
GET CHECKED BEFORE MOVING ON
6
4.
5.
There are five objects positioned in front of the
plane mirror in the drawing to the right. Which
images will the observer be able to see as he
looks at the mirror? This will be a hard problem
for most people. In order to say that a particular
image can be seen, the most direct way is to
show a light ray’s path, starting at the object and
then reflecting appropriately into the pupil of the
eye. There are two other clever ways as well.
Show appropriate rays to prove your conclusion.
Plane Mirror
A
B
D
E
C
The diagram below consists of three mirrors. Draw a ray diagram to locate the object
producing the indicated image as seen by the eye.
Image
Mirror
7
GET CHECKED BEFORE MOVING ON
6.
What is the minimum length of a plane mirror used to fully view your body, assuming the mirror will be
vertical, untilted, and in front of you? Explain and show a diagram below.
7.
The cat below is sitting in front of a plane mirror. Carefully follow the following instructions in order to
show how the cat’s image is formed by the plane mirror.
a. Draw two rays from the tip of the ear of the cat to the mirror and two rays from the tip of the paw of the
cat to the mirror.
b. Use the law of reflection to draw the four reflected rays.
c. Trace each of the reflected rays back behind the mirror (these should be dashed lines since they are only
apparent paths.
d. Indicate the positions of the images of the cat’s ear and paw at the point where the apparent paths appear
to converge (or emerge from).
Plane Mirror
8