LO - Lab #05 - How are images formed from light?

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June Watkins
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1 LO - Lab #05 - Helpful Definitions: The normal direction to a surface is defined as the direction that is perpendicular to a surface. For example, place this page flat on the table and then stand your pencil so it is straight up and down (sticking up out of the table). Your pencil is now along a normal line to the surface of the page. Task #1 - How can you make light change its direction? Equipment: Ray box with slit inserts, 1 Plane mirror, 1 Acrylic rectangular piece, Protractor. Using a Plane Mirror Adjust the ray box to produce one thin beam of light. Place the plane mirror and ray box on your lab notebook. Clearly mark the front edge of the mirror and its normal. Shine the light onto the mirror so that it forms an angle of about 25 with the normal. Carefully trace (using a straight edge) the path(s) of the light in your notebook. Measure and record the angle between the surface and the incident beam of light, between the incident beam of light and the surface normal (the incident angle), between the reflected beam of light and the surface normal (the reflected angle), and between the reflected beam of light and the surface. Repeat the experiment enough times so that each person in your group has a drawing in his or her logbook. Use a different incident angle for each trial. All group members should record the angle measurements for each case. 1. Based on your experimental evidence and previous experience, discuss the following and record your answers: Where does the light bend while interacting with a mirror? Be specific. If the reflecting material were thicker (as in wider, not as in taller), would it change the way the light is bent? Why or why not? Do your data support the law of reflection? Why or why not? Be specific! In words, describe the relation between the incident and reflected angle for a plane mirror. Page 1 of 13

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3 Using an Acrylic Rectangular Block Adjust the ray box to produce one thin beam of light. Place the rectangular acrylic piece and ray box on your lab notebook. Clearly mark the front edge of the piece and its normal. Shine the light onto the edge of the acrylic piece so that it forms an angle of about 25 with the normal. Carefully trace the path(s) of the light in your notebook. Measure and record the angle between the incident beam of light and the surface normal (the incident angle) and between the refracted beam of light and the surface normal (the refracted angle). You can ignore any reflected beams of light since you already studied reflection. Repeat the experiment enough times so that each person in your group has a drawing in his or her logbook. Use a different incident angle for each trial. All group members should record the angle measurements for each case. θ 1 θ 2 2. Based on your experimental evidence and previous experience, discuss the following and record your answers: Where does the light bend while interacting with a transparent material? Be specific. If the refracting material were thicker (like a block that is twice as wide), would it change the way the light is first bent? Why or why not? Do your data support Snell's law of refraction (n1 sinθ1 = n2 sinθ2)? Why or why not? Be specific! If nair = 1.000, then what is nacrylic? In words, describe the relation between the incident and refracted angle for a transparent substance. Page 3 of 13

4 3. How is the behavior of light interacting with a plane mirror similar to and different from the way light interacts with a material like the acrylic block? Page 4 of 13

5 Task #2 - Where do you see the image? Using a Plane Mirror Equipment: Ray box with slit inserts, 1 Plane mirror, Cardboard square, 2 Colors of pushpins, Newsprint, Meter stick, Protractor. This activity is very important, but it is also kind of tricky. You need to learn what to look for, but you can only do this by trying it out. Everyone needs to do the following activities. Your instructor will help as much as he/she can, but in the end only you can see through your eyes and only you can do these activities for yourself. Place a sheet of clean newsprint on top of a large piece of cardboard. Stand the plane mirror upright in the center of the paper. On the paper, draw the location of the front edge of the mirror. Pick a spot in front of the mirror and stick a pushpin through the newsprint and into the cardboard base. Mark this spot on the newsprint. This will be the object pushpin. 4. Based on your life-long experience with plane mirrors, predict where the pushpin's image will be. Describe the basis for your prediction. Part #2-A: Try the following experiment to test your prediction: Pick a spot in front of the mirror to put your eye for viewing (this spot should be different than where you put the object pushpin). Label this location on the newsprint. View the image of the pushpin in the mirror from this spot. Leaving the object pushpin in place, pick up a second pushpin. Using one eye to look at the image of the object pushpin and the other eye to look at the second pushpin, put the second pushpin in the location where your eye perceives the image to be. If you have trouble seeing this, you might try having your lab partner move their finger around until it appears to be pushing down on the image of the pushpin. Then they can stick the second pushpin in this location. Mark the location of the second pushpin and write the observer's name by the location. Ask your instructor for feedback before you continue. Make sure you do this activity correctly! Have your instructor inspect your first trial before you move on. Pick a new observer and give that person the second pushpin. Repeat Part #2-A (keeping the same piece of newsprint and without moving the object pushpin) by putting the new observer's eye in a different viewing spot in front of the mirror. Page 5 of 13

6 Each member of your group must serve as the observer, using different viewing locations. Be sure to mark the location of the observer's eye for each trial. Page 6 of 13

7 In your logbook, carefully draw the path that the light actually takes in going from the object pushpin to your eye. Recall your law for the reflection of light as you answer this question. Be sure to represent the precise position of your eye in your drawing. You should use the following symbol to represent the eye. Opening to your eye Retinal i mage formed here 5. Where do these rays appear to come from? Where do they actually come from? 6. Where is the image located in relation to the mirror and/or your eye? Explain. How does this compare to your answer to #4? The distance from the object (pushpin) to the mirror's reflecting surface is called the object distance (s0) and the distance from the image to the mirror's reflecting surface is called the image distance (si). Measure and compare these two quantities for your data. 7. What conclusions can you make about the position of the object and the position of its image for a plane mirror? Using Refracting Lenses Equipment: 2 Glass lenses in holders (+100, -150), Optics bench, Pasco light box with an arrow pattern. This activity is also important, but it is again kind of tricky. You need to learn what to look for, but you can only do this by trying it out. Everyone needs to do the following activities. Your instructor will help as much as he/she can, but in the end only you can see through your eyes and only you can do these activities for yourself. Hints for seeing an image in Parts #2-C and #2-D Stand up and move so your body is in line with the optics bench and is past the end of the optics bench. Try moving your eyes up and down a bit to help recognize where the image appears to be. Have a lab partner move their finger until it appears to be just above or aside of the image location. Be sure you do not look at this finger through the lens. Pretend like you need to grab the image with your hand. Page 7 of 13

8 Do not stare at the bright light too long if it bothers your eyes. Page 8 of 13

Part #2-C Securely place a light box with an arrow pattern on the optics bench. Note its position on the provided scale. Start with the +100 lens, which has a curvature that is shaped like this.

9 Part #2-C Securely place a light box with an arrow pattern on the optics bench. Note its position on the provided scale. Start with the +100 lens, which has a curvature that is shaped like this. This is called a double-convex lens or a converging lens. Position the lens on the optics bench 30 cm in front of the light box's arrow pattern. The arrow pattern on the light box will serve as the "object" for this activity. Place your eye (and body!) well beyond the position of the image of the arrow pattern (past the end of the optics bench) and look back toward the lens. Identify the location of the image of the arrow pattern as seen by your eye. Describe the size and orientation of the image as compared to the original object. Note, the size of the image (yi) is measured from the top to the bottom of the image, not the height above the table! y0 yi s0 si Measure and record the object distance (s0) between the light box and the lens and the image distance (si) between the lens and the position at which you see the image. When recording data for thin lenses, be sure to use the sign conventions given at the bottom of this page and as outlined in your textbook (page 869). Carefully sketch a ray diagram that includes the object, lens, image, and depiction of your eye. Label all relevant distances in your diagram including the focal point. Try to make your drawing approximately to scale. 8. According to the thin lens formula, the focal length of a lens can be determined by: 1 = Estimate the focal length, f, of this lens using your data. Compare the f s 0 s i calculated focal length to the value listed on the lens holder. Sign Conventions for Thin Lenses s0 is + if the object is in front of the lens. s0 is - if the object is in back of the lens. Page 9 of 13

10 si is + if the image is in back of the lens. si is - if the image is in front of the lens. f is + for a converging lens. f is - for a diverging lens. yi is + for an upright image. yi is - for an inverted image. Page 10 of 13

11 Part #2-D Remove the lens, but keep the light box in the same position. Start with the -150 lens, which has a curvature that is shaped like this. This is called a double-concave lens or a diverging lens. Position the lens on the optics bench about 20 cm in front of the light box. Place your eye (and body!) well beyond the position of the image (past the end of the optics bench) and look back toward the lens. Identify the location of the image as seen by your eye. This can be tricky so you may have to try a couple of times. Remember, if you use your finger to help gauge the distance, do not look at the finger through this lens as that will distort your perspective. Describe the size and orientation of the image as compared to the original object. si s0 Measure and record the object distance (s0) and the image distance (si) for the image that you see. Be sure to use the given sign conventions with your numerical values. Carefully sketch a ray diagram that includes the object, lens, image, and depiction of your eye. Label all relevant distances in your diagram including the focal point. Estimate the focal length of this lens using your data and the thin lens formula. 9. Compare the calculated focal length to the value listed on the lens holder. 10. Compare the behavior of these two kinds of lenses as they interact with light. Be as specific as possible. Why do you think the names converging and diverging are often used to describe these lenses? Page 11 of 13

12 Types of images: An image that can actually be displayed on a screen placed at the image position is called a real image. The rays of light actually pass through the image for a real image. An image that cannot be displayed on a screen placed at the image position is called a virtual image. The rays seem to emerge from a point on the other side of the lens. The rays do not actually pass through the image position in a virtual image. The fact that the human eye can see real and virtual images is pretty remarkable! As you have hinted with your ray diagrams, a real image of the original real or virtual image is formed on your retina. 11. Based on the available equipment and your experience, which of the following produce real images and which produce virtual images? Plane mirror Converging lens Diverging lens Be specific in describing your evidence for your conclusions. 12. In words, describe what it takes for a ray of light to change directions. Task #3 - What is the effect of lenses with more or less curvature? Equipment: Glass converging lens in holder (+200), Optics bench, Pasco light box with an arrow pattern, White viewing screen. Complete the following experiment with the +200 converging lens: Select a new object distance. Leaving the light and lens fixed, adjust the viewing screen until you are able to obtain a clear image. Make a data table and record the height of the object (y0), the height of the image (yi), whether it is inverted or upright, the object distance (s0) between the light box and the lens, and the image distance (si) between the lens and the screen. Repeat this for a second object position. 13. Do your data support the thin lens formula? Explain why or why not. Page 12 of 13

13 14. Which lens has the greater curvature (+200 or +100)? Does the curvature of a lens seem to affect how it is able to bend light? Write a conclusion of how the focal length and lens curvature appear to be related. Page 13 of 13

Part 1 Investigating Snell s Law

Part 1 Investigating Snell s Law Geometric Optics with Lenses PURPOSE: To observe the refraction of light off through lenses; to investigate the relationship between objects and images; to study the relationship between object distance,