Name (printed) _______________________________
LAB
REFLECTION FROM A PLANE MIRROR
INTRODUCTION
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.
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).
Mirror
Reflected Ray
Reflected
Angle
Incident
Angle
Incident Ray
Normal
The ray that initially strikes a mirror is the incident ray. After it reflects it is then the
reflected ray. At the point of reflection, a line drawn perpendicular to the surface of the
mirror is the normal. The incident angle is always measured between the incident ray and
the normal and the reflected angle is always measured between the reflected ray and the
normal.
Figure 1.6: Reflection terminology
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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.
Place cardboard under the next page of the handbook.
2.
Place the reflective (back) edge on the line marked “mirror”
on the next page.
3.
Place the florist pin on the point marked “object.”
4.
Place the straight pins on the points “1a” and “1b.”
5.
Note how when you get your eye level with
the paper and look across the page so that the
straight pins are lined up, the image of the
florist pin also lines up with them
(Figure 1.7a).
6.
Look into the mirror from the right side and
place the straight pins so they line up with the
image of the florist pin, as before
(Figure 1.7b). Label these points “2a” and
“2b.”
7.
Repeat this for a third angle.
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 of the
florist pin to the eye. (I’ve done this for the
first set of pins).
9.
Use the intersection of these dotted lines to
label the image position with the word
“image.”
10. For each of the three sets of points, draw a
solid line from the florist pin to the mirror (at
the point the dashed line intersects the mirror)
and then down to the straight pins. This
represents the path that light actually takes as
it goes from the object (florist pin) to the eye.
Figure 1.7a: View into the mirror of the two
stickpins lined up with the image of the florist pin.
Figure 1.7b: Placing the pins to locate an additional
apparent path of light from the image to the eye.
11. Draw a perpendicular line (normal) to the
mirror at the points where the solid lines
touch the mirror.
12. Measure and record the three sets of incident
and reflected angles.
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LAB DRAWING
Mirror
1a
1b
Object
DATA
Record incident and reflected angles in the table below.
Sighting
∠i
∠r
1
2
3
3
PLANE MIRRORS
QUESTIONS AND PROBLEMS
1.
How does the size of the incident angles compare to the corresponding reflected angles?
2.
The image and object distances are the shortest distances from the object and image to the mirror. What are
these distances (in cm) and how do they compare to each other?
GET CHECKED
BEFORE MOVING ON
3.
A ray of light strikes a plane mirror at an angle of 28° from the mirror. What is the angle between the
incident and reflected rays? Draw a diagram as part of your explanation.
4.
Let’s say you were standing 2.0 meters in front of a plane mirror and facing it. And let’s say that a very
attractive person is standing 4.5 meters behind you. You have your camera and want to take a picture, but
you’re too shy. So you decide to take a picture of the person’s image and point your camera toward the
mirror. What distance from you would you have to focus your camera in order to have the image of your
crush in clear focus? Explain clearly and draw a diagram as part of your explanation.
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
4
A
B
D
E
C
6.
7.
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.
There are three objects positioned near a plane mirror in the drawing below. Which one(s) will the observer be
able to see reflected in the mirror?
a
b
c
GET CHECKED
BEFORE MOVING ON
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8.
The cat below is sitting in front of a plane mirror. Draw two rays from the ear of the cat to the mirror and
two rays from the foot of the cat to the mirror. Use the law of reflection to draw the four reflected rays.
Finally, determine the position of the cat’s image by tracing these back to the points from which they
appear to emerge.
Plane Mirror
9.
The cat below is sitting in front of a convex mirror. Draw two rays from the ear of the cat to the mirror and
two rays from the foot of the cat to the mirror. Use the law of reflection to draw the four reflected rays.
Finally, determine the position of the cat’s image by tracing these back to the points from which they
appear to emerge.
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