Download Images Formed by Plane Mirrors

SCIENCE EXPERIMENTS ON FILE™ Revised Edition
6.16-1
Images Formed by Plane Mirrors
Donald A. Schaefer
Topic
Laws of reflection
Time
1 hour
!
Safety
Please click on the safety icon to view the safety precautions. Do not use regular
glass.
Materials
two sheets of Plexiglas™, each about
1-ft square
four heavy bookends
two identical candles
two identical candleholders
ruler
one string of multicolored
Christmas tree lights
tape
Procedure
PART A: REFLECTION
IN A
SINGLE PLANE MIRROR
1. Using two of the bookends, stand a sheet of Plexiglas™ up on its side in the middle
of a table, with two bookends on one corner holding it vertical, as shown in figure 1.
Light one of the candles and place it in front of the sheet. This is the “object” candle.
Dim the lights. Look through the Plexiglas™ sheet to the place where the candle appears to be and place a second candle there. Notice that your “image” candle appears
to be lighted, even though it is not. Light the second candle.
2. Stay on the object candle side of the mirror. Move to your left and then to your
right while looking at the reflected image of the first candle superimposed on the
second candle. Notice that the image itself disappears when you move too far left
or right. Does the second candle always seem to be perfectly aligned with the reflected image of the first candle?
3. Measure the distance from the object candle to the Plexiglas™. To which surface
of the Plexiglas™ should you measure? Why that surface? Try bringing your pencil up next to the mirror surface. Does your pencil top seem to touch its image, or
is there a small space between them? What does this observation tell you? Measure the distance from the image candle to the Plexiglas™. Record your measurements Do (object distance) and Di (image distance) on the data table.
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6.16-2
SCIENCE EXPERIMENTS ON FILE™ Revised Edition
Figure 1
Plexiglass™ sheet
candle
bookend used to support sheet
D ATA T A B L E
Single mirror
Position
D0
Di
Position 1
Position 2
Double mirrors
Angles
D0
Di
Angles
you chose
less than 90°
4. Move the object candle closer to the sheet, and adjust the image candle so it
again appears to be perfectly aligned with the image, even when you move from
left to right. Repeat steps 2 and 3. Can you conclude that there is a definite relationship between D0 and Di values? Draw a diagram of the mirror, the candle, the
reflected image, and where your eye is located when you make observations for
one particular object/image arrangement. Draw three “lines of sight” in your diagram that show the path the light took from the image to the mirror to your eyes
for each of the three eye locations. Where did the light really come from as it
seemed to make an image of the object candle? Can you draw the complete path
for the light—from object to mirror surface to eye? Do you see any relationship
between the angles of light paths from object candle to the reflecting surface and
from the reflecting surface to your eye?
5. Blow out both candles. Attach the Christmas tree lights with tape to the top of
one of the candles so that there is a different-colored bulb on either side of the
candle top, as in figure 2. Plug the lights in, and place the candle in front of the
sheet. What happens to right and left in the reflected image?
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SCIENCE EXPERIMENTS ON FILE™ Revised Edition
6.16-3
Figure 2
lights attached to candle with tape
PART B: MULTIPLE IMAGES
IN
PLANE MIRRORS
1. Move the candle to the side and set up the second sheet of Plexiglas™ so that the
sheets are at right angles to one another and touch along one edge, as shown in
figure 3. Before you place the candle with the lights on it in front of the sheets,
predict how many images you will see.
Figure 3
two sheets at 90-degree angle
candle
2. Put the candle in front of the mirrors, slightly closer to one mirror than to the
other. How many images do you see? Measure Do from the object candle to the
junction of the two mirrors. Place the second candle over each reflected image in
turn and measure each Di. Record these values on the data table. What can you
say about the Do and Di values? What happens to left and right in each image?
Why? Try to draw a complete diagram of the mirrors, one location for the candle
and its images, including lines of sight that explain how you see each image.
3. Move the mirrors so that the angle between them is less than 90 degrees. What
happens to the location and the number of images? Continue to narrow the
angle, noticing where the images move, how many there are, and what happens
to left and right in each one. Measure each Do and Di for each new angle. Always
measure to the junction of the two mirrors.
4. What conclusion can you draw about the relation of the distance from the object
to the reflecting surface and the distance from the image to the reflecting surface?
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6.16-4
SCIENCE EXPERIMENTS ON FILE™ Revised Edition
5. Were left and right reversed in the reflected image in one mirror? In the images
in two mirrors at a right angle? In the images in two mirrors at narrowing angles? Can you explain why? Referring to your diagrams and notes, what is the
order in which the reflections are reversed and not reversed?
What’s Going On
The candle placed over the reflected image will always appear perfectly aligned with
the image, no matter from what angle you view it. The distance from the first candle to the Plexiglas™ and the distance from the second candle (reflection of the first
candle) to the Plexiglas™ are equal, but the measurement must be made to the reflecting (front) surface. This will be true no matter where the first candle is placed.
This is an example of the law of reflection, which states that a reflected image will
always appear as far behind the mirror as the object is in front of the mirror.
Figure 4
B
A
light
I
R
light
distance A = distance B
point of observation
angle I (incidence) = angle R (reflection)
Your diagram of the candle and reflection will vary somewhat according to the
positions of the candle and the viewer, but it should always show the same basic relationships among object, reflection, mirror, and viewer, as in figure 4. In particular,
the angle between the incident light ray and the mirror is always equal to the angle
between the reflected ray and the mirror. Right and left are reversed in the image in
the single mirror. You see three images in the mirrors at 90 degrees. Two are reversed
right and left and the third—called a secondary image—is not (figure 5). This is because it is a reflection of a reflection, so it has actually been reversed twice, once in
the first reflection and then again in the reflection of the reflection. In the case of
mirrors less than 90 degrees apart, the images alternate—whenever an odd number
of reflections has occurred, the image is reversed; whenever an even number, the
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SCIENCE EXPERIMENTS ON FILE™ Revised Edition
6.16-5
image is not reversed. The narrower the angle, the more images appear (figure 6). Do
and Di, are equal for all angles of the mirrors. If you place the pivot point of a compass at the junction of the mirrors and place the lead point on the object, you can
draw a circle that will pass through every image. They all “stand around in a circle”
whose radius is the distance from the junction of the mirrors to the object
Figure 5
Figure 6
.
source
light
point of observation
Connections
Optical laws of reflection underlie the design and operation of optical instruments,
including cameras, microscopes, and telescopes. In this project you investigated the
laws governing reflection in plane (flat) mirrors.
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Safety Precautions
READ AND COPY BEFORE STARTING ANY EXPERIMENT
Experimental science can be dangerous. Events can happen very quickly while you are performing
an experiment. Things can spill, break, even catch fire. Basic safety procedures help prevent serious
accidents. Be sure to follow additional safety precautions and adult supervision requirements for
each experiment. If you are working in a lab or in the field, do not work alone.
This book assumes that you will read the safety precautions that follow, as well as those at the start
of each experiment you perform, and that you will remember them. These precautions will not always
be repeated in the instructions for the procedures. It is up to you to use good judgment and pay attention when performing potentially dangerous procedures. Just because the book does not always
say “be careful with hot liquids” or “don’t cut yourself with the knife” does not mean that you should
be careless when simmering water or stripping an electrical wire. It does mean that when you see a
special note to be careful, it is extremely important that you pay attention to it. If you ever have a
question about whether a procedure or material is dangerous, stop to find out for sure that it is safe
before continuing the experiment. To avoid accidents, always pay close attention to your work, take
your time, and practice the general safety procedures listed below.
PREPARE
• Clear all surfaces before beginning work.
• Read through the whole experiment before you start.
• Identify hazardous procedures and anticipate dangers.
PROTECT YOURSELF
• Follow all directions step by step; do only one procedure at a time.
• Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and
first-aid kit.
• Make sure that there is adequate ventilation.
• Do not horseplay.
• Wear an apron and goggles.
• Do not wear contact lenses, open shoes, and loose clothing; do not wear your hair loose.
• Keep floor and work space neat, clean, and dry.
• Clean up spills immediately.
• Never eat, drink, or smoke in the laboratory or near the work space.
• Do not taste any substances tested unless expressly permitted to do so by a science teacher in
charge.
USE EQUIPMENT WITH CARE
• Set up apparatus far from the edge of the desk.
• Use knives and other sharp or pointed instruments with caution; always cut away from yourself
and others.
• Pull plugs, not cords, when inserting and removing electrical plugs.
• Don’t use your mouth to pipette; use a suction bulb.
• Clean glassware before and after use.
• Check glassware for scratches, cracks, and sharp edges.
• Clean up broken glassware immediately.
v
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Safety
SCIENCE EXPERIMENTS ON FILE™ REVISED EDITION
• Do not use reflected sunlight to illuminate your microscope.
• Do not touch metal conductors.
• Use only low-voltage and low-current materials.
• Be careful when using stepstools, chairs, and ladders.
USING CHEMICALS
• Never taste or inhale chemicals.
• Label all bottles and apparatus containing chemicals.
• Read all labels carefully.
• Avoid chemical contact with skin and eyes (wear goggles, apron, and gloves).
• Do not touch chemical solutions.
• Wash hands before and after using solutions.
• Wipe up spills thoroughly.
HEATING INSTRUCTIONS
• Use goggles, apron, and gloves when boiling liquids.
• Keep your face away from test tubes and beakers.
• Never leave heating apparatus unattended.
• Use safety tongs and heat-resistant mittens.
• Turn off hot plates, bunsen burners, and gas when you are done.
• Keep flammable substances away from heat.
• Have a fire extinguisher on hand.
WORKING WITH MICROORGANISMS
• Assume that all microorganisms are infectious; handle them with care.
• Sterilize all equipment being used to handle microorganisms.
GOING ON FIELD TRIPS
• Do not go on a field trip by yourself.
• Tell a responsible adult where you are going, and maintain that route.
• Know the area and its potential hazards, such as poisonous plants, deep water, and rapids.
• Dress for terrain and weather conditions (prepare for exposure to sun as well as to cold).
• Bring along a first-aid kit.
• Do not drink water or eat plants found in the wild.
• Use the buddy system; do not experiment outdoors alone.
FINISHING UP
• Thoroughly clean your work area and glassware.
• Be careful not to return chemicals or contaminated reagents to the wrong containers.
• Don’t dispose of materials in the sink unless instructed to do so.
• Wash your hands thoroughly.
• Clean up all residue, and containerize it for proper disposal.
• Dispose of all chemicals according to local, state, and federal laws.
BE SAFETY-CONSCIOUS AT ALL TIMES
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