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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. © Facts On File, Inc. 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? © Facts On File, Inc. 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? © Facts On File, Inc. 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 © Facts On File, Inc. 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. © Facts On File, Inc. 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 © Facts On File, Inc. vi 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 © Facts On File, Inc.