Light Up the Room with a Light Bulb Illusion. By Laura Zinszer

Transcription

1 Light Up the Room with a Light Bulb Illusion By Laura Zinszer lzinszer@ .com

2 Summer Institute Optics and Sound with Dr. Meera Chandrasekhar and Jennifer Geib Light Up the Room with a Light Bulb Illusion By Laura Zinszer lzinszer@ .com

3 Teacher Pages What is the concept? When an object is placed in front of a concave mirror at the center of the sphere, an inverted, real image will be created which is the same size of the original object. How does the activity illustrate the concept? This activity is used to illustrate how a concave mirror can be used to reflect and bend light to create the illusion of a solid object. A light bulb seems to appear in a light socket and light up a room. A hand, waving over the light socket appears to move right through the light bulb while it is working. A real image is created which is inverted and the exact size of the original light bulb. What are the qualitative and quantitative application? Rules of reflection 1. When a ray travels parallel to the axis, it is reflected back through the focal point. 2. A ray which travels through the focal point is reflected back parallel to the axis. 3. A ray which travels through a normal through the center of the sphere will be reflected back through the normal. Light Bulb Illusion 46.3 cm Height of mirror from table to the top of frame Real image 68.5 cm Axis from the center of the mirror to the center of light bulb 40 cm Diameter of concave mirror F cm Focal point Table top 24.6 cm Light bulb 17 cm 24.9 cm

4 Important Measurements for this Activity o (distance object is from mirror)= 68.5 cm C (center of sphere of mrror) = 68.5 cm i (distance image is from the mirror) = 68.5 cm d (diameter of mirror) = 40 cm Image and Focal Point The focal point of a concave mirror is found when the inverse of the distance of the image from the mirror is added to the inverse of the distance of the object from the mirror. The focal point for this mirror is cm from the center of the mirror. Because the image distance from the mirror is a positive number, this indicates a real image will be created. 1/F = 1/i +1/o 1/F = 1/68.5 cm + 1/68.5cm 1/F = 2/68.5 cm 1/F = 1/34.25 cm F = cm In this activity, the focal point is found along the axis, exactly half-way between the mirror and the light bulb. This occurs because the light bulb was placed at exactly the center point of the sphere for the mirror. According to the first rule of reflection, a ray that travels from the light bulb parallel to the axis, is reflected back through the focal point. The second ray travels toward the mirror through the focal point, and is reflected back parallel to the axis. The spot where these two rays cross is the focal point. Because the object was placed at the center of the sphere, the rays from the object will be reflected from the mirror and cross at a focal point exactly halfway between the mirror and object. Mirror Ray 2 F Ray 3 C object Ray 1

5 Magnification The magnification for a concave mirror is determined by dividing the negative distance of the image from the mirror, by the distance of the object from the mirror. The o and the i are both 68.5 cm. In this activity, the magnification was equal to a factor of -1. When the image distance is equal to the object distance, the real image created will be equal in size to the object. The magnification was a negative number which indicates that the image will be inverted. m = -i / o m = cm/68.5 cm m = -1 Real image Light bulb The on-line resources for this activity are found in the Exploratorium. The web-site is

6 How to build this project? Building the Mirror Front Plywood for Mirror Frame Cut a 45 cm square piece of _ in plywood. Take a 40 cm concave mirror and trace a circle round the mirror with a pencil. Draw a second circle 1 cm inside the first circle. Use a jig saw and cut out the inside circle. 1 cm 40 cm diameter Cut a second piece of 3/8 in plywood, 45 cm square. Place the 1 st square plywood on the second piece and trace the inner circle. Use the jigsaw to cut out the second circle at a 30* angle so that the mirror will sit in the groove. Use a flat, black spray paint and paint each plywood square. Place the two plywood squares together and predrill 12-1/8 in holes in the corners of the wood. Run a bead of clear acrylic caulk around the angled groove and place the mirror on the groove. Put the caulk on the plywood, and put the other piece of plywood over the mirror. Use _ in. black wood screws to seal the mirror between both pieces of plywood. Cut a base that is 45 cm wide and 30 cm deep from 1 inch plywood. Cut two 90* angle braces that are 30 cm tall and 20 cm at the base. Paint the baseand angle braces black. Use eight, 3 inch wood screws to secure angle braces to the base and the mirror frame to the base. 45 cm square Jig saw Back Plywood for Mirror Frame Back Plywood for Mirror Frame Front Plywood for Mirror Frame Mirror Base 40 cm concave mirror

7 Building the Light Box The light box is made of 1.3 cm x 20 cm wide, pine plywood. Use a jigsaw and cut out 5 pieces of plywood for the box. Cut a rectangular base that is 16.3 cm deep x 15 cm wide. Cut two identical sides that are 15 cm deep x 22.3 cm tall. Cut a back piece that is also 15 cm wide x 22.3 cm tall. The top for the box is screwed on last. It needs to be identical to the base, 15 cm wide x 16.3 cm deep. Use wood glue and 1 inch, black wood screws. Use a 1/8 inch drill bit and predrill the sides on the base. Put glue on the base and screw the back and sides to the base. Predrill holes in the top of the light box and glue and screw down the top on the box. Paint the box black and leave for one hour. Apply a second coat of paint to the box so that no grain is evident. Drill a hole in the base for the light cord. Drill a hole in the side of the box, near the top for the pull switch. 1.3 cm Electrical Cord 1.3 cm Front of Light Box 15 cm Back of Light Box Pull Switch 22.3 cm 1.3 cm 22.3 cm Position the light sockets so that they will be on the box opposite one another. Place one socket on top of the box and mark the holes for the screws. Predrill the holes so that they parallel the sides of the box. Place the interior socket in the box so the pull cord will move out the the hole in the side of the box, and predrill these holes so that they are about 30* off normal. 1.3 cm 15 cm Side of Light Box 1.3 cm Thread the electrical cord through the hole in the base. Wire the interior socket with the electrical cord. Do not wire the external light socket. It would be very dangerous and is not necessary for this demonstration. Use black electrical tape to camaflauge the cord inside the box along the wall. Finally, use black electrical tape to connect both pull cords through the side of the box. 1.3 cm 15 cm 16.3 cm 22.3 cm 1.3 c

8 Activity Instructions Introductory Prodedure: 1) Set up the mirror and light box before class begins. Darken the room and have the students walk in and move to the back of the room so that they can observe the illusion. 2) Take your hand and move it through the image so it appears to the students that you have moved through s solid object. 3) Turn off the light box so the students can observe that no light bulb was in the upper socket. Examining the Discrepant Event: 1) Have the students move to their seats without letting them look into the box. Hand out activity sheets to record observations. 2) Ask the students to write down and draw what they observed. Have they included the mirror in their drawing? Did they observe anything special about the mirror? What is the best way to portray the lightbulb image? 3) With a partner, have the students try to make a rule to explain their observations. 4) Have the students share their ideas with the class, and let them challenge, question, or add to the thoughts of the class. Use a large piece of writing paper to list the class ideas to explain this phenomenon. 5) Now, have the students predict what the inside of the box might look like. With their partner, draw their best guess to describe the inside of the box. 6) As a class, record and draw the different descriptions for the inside of the box. Have the students share their ideas with the class, and let them challenge, question, or add to the thoughts of the class. Use another large piece of writing paper draw a class description of the inside of the box. 7) Now, turn the box around to the class and let them examine the inside of the box and the mirror up close. Questions to Help Examine the Discrepant Event: 1) What surprised them and what observations matched their ideas and drawings? 2) What additional observations can the students make about the mirror? 3) How is the box alike or different from their first ideas? 4) Why do they think that the mirror is shaped the way it is? 5) Would this activity work with a flat mirror? A convex mirror? Clear glass? Why or why not? 6) Would this activity work with the light shining on a different type of surface like paper or cardboard? Why or why not?

9 Understanding the Discrepant Event: 1) Now, have the class reset the activity. Select a box controller and and mirror operator. Have the class give the two operators instructions to recreate the illusion. 2) Try setting up the illusion with the box very close to the mirror, have the students describe what they observe. 3) As the operators move the box away from the mirror, have the students describe what they observe. Questions to Guide Understanding of the Discrepant Event: 1) How do we know when the box is too far from the mirror to create the illusion? 2) What rule can we make about the image when the box is too far away to create the illusion? 3) How do we know when the box is too close to the mirror to make the illusion? 4) What rule cn we make when the box is too close to the mirror to make the illusion? 5) Finally, how do we know when the box is just the right distance to the mirror to make the illusion? 6) What rule can we make when the box is positioned exactly the right distance to the mirror to make the illusion?

10 Student Pages 1. What did you observe about the light demonstration? Write three or four observations here, and draw what you observed. 2. What beginning rule can you make that might explain your observations?

11 3. What do you think the inside of the box looks like? Draw your ideas in this box here. What is the reason for drawing the box the way you did? 4. Now, look inside the box, and draw what you observe. Draw your observations in this box here. How is this drawing different from your prediction?

12 5. What rule can we make when the box is too far from the mirror to make the illusion? 6. What rule can we make when the box is too close to the mirror to make the illusion? 7. What rule can we make when the distance the box is from the mirror is just right to make the illusion?

13 Materials List Number Material Obtain From Cost 1 45 cm sq. _ inch plywood Hardware store $ cm sq. 3/8 inch plywood Hardware store $ cm Spherical Mirror Sargent and Welch $ inch wood screws Hardware store $ inch black wood screws Hardware store $ feet _ x 8 inch quality plywood Hardware store $ _ inch black wood screws Hardware store $ Container clear acrylic caulk Hardware store $ Single light bulb sockets Hardware store $ Electrical cord Hardware store $ Black electrical tape Hardware store $ Watt bulb Hardware store $ Extension cord Hardware store $ Flat, black, acrylic spray paint Hardware store $5.00 You also need: a cordless drill with 1/8 bit, _ inch bit, and phillips head screwdriver a jigsaw with blade for cutting plywood