Cabrillo College Physics 10L Name LAB 12 Reflection and Refraction Read Hewitt Chapters 28 and 29 What to learn and explore Please read this! When light rays reflect off a mirror surface or refract through a lens, they change their direction of travel, thus fooling our eye/brain combination into thinking they came from someplace different than their true source. We call the true source of light the object and the place our brain thinks the light came from the image. The images we see in mirrors and with a magnifying glass are called virtual images, since they can only be seen by looking "through" a lens or mirror. They look very real, but the light isn t really there where the image seems to be. Other images can be projected onto a surface these are called real images but they are sometimes difficult to see without the help of a screen. We will meet both types of images in this lab. We can create images where we want them by varying the curvature of the mirror or lens. We describe the mirror or lens by its focal length, and you will find that more strongly curved surfaces result in shorter focal lengths. The focal length is the distance from the lens or mirror to the spot where a group of parallel rays of light come together. Terms to know: Reflection: light rays bounce off a surface: specular (light rays stay together in a beam, as in mirror reflection) or diffuse (light rays scatter randomly, as off a white sheet of paper) Refraction: light rays bend at the boundary when they enter or exit a material Virtual image: where your brain thinks the image is located in a mirror or through a lens Real image: where light rays converge to form an image that can project onto a screen Focal length: the distance from a lens or mirror where parallel incoming rays come together What to use Concave, convex, and plane mirrors, lots of lenses, optical bench, light ray box, protractors and straight edges, scratch paper, lasers, transparent water containers, optical illusion banks, fiber optics demos. What to do Experiment with the equipment provided to help answer the questions I have asked below and other questions of your own. Mandatory Comments Please use this space for comments and suggestions for improving this lab. Thank you. 93
1) Focal Points of Mirrors and Lenses (use the mirrors and lenses from the blue box) Adjust the ray box to give you five parallel white rays. a) Shine the rays straight into the concave mirror (shaped like a cave ) so the light is reflected back toward the ray box. The place where the five reflected rays cross each other is the focal point of the mirror. The distance between the focal point and the mirror s surface is called the focal length (f) What is the focal length of this mirror? f of mirror = cm. Make a freehand sketch below of the mirror and light rays that you used to measure the focal length. b) Shine the rays through the convex lens from your kit (the one that is fatter in the middle than at the edges) and find the focal point like you did above. Make a sketch of the lens and the light rays. What is the focal length of this convex lens? f of lens = cm. c) Try a concave lens thin in the middle. What do the parallel light rays do when they go through it? How might you measure its focal length? Do you see why we call this a negative focal length? Make a sketch of the concave lens and the light rays. a) Predict what will happen if you nest the concave and the convex lenses together. Sketch what you think will happen to the parallel light rays. Then try it out. Were you right? 94
2) Images from Lenses Use these lenses ->> a) Very distant object focal length Use a convex lens to form a clear image on a screen of a distant bright object. How far is the image from the center of the lens? This is the focal length of the lens. Is this image real or virtual? How do you know? (Hint: can it be projected on a screen?) Is the image right side up or inverted? b) Projector (use the 100mm lens for this activity) Place a bright light source (on the side of ray box) near your lens, about one focal length from it. Try to project an image onto a nearby wall or piece of paper. Move the lens in and out to focus the image. Play with it! Try to make a large projected image what did you need to do? Is the image upright or inverted? c) Magnifying Glass Hold your convex lens very close to this piece of paper and read this: what s up? Move the lens very slowly away from the paper. What happens to the image? How far from the paper can you hold the lens and still read the words? Is this more or less than the focal length of the lens? Try this also with a concave lens (the one marked -150mm) What happens? d) Making a Beam When a beam of parallel light strikes a converging lens (or mirror), the light converges at the focal point. What happens if we do the opposite: put a bright light source at the focus? Try it. (It works well with a 100mm lens). Make a little sketch showing what the light rays do. Name some devices that use this principle. 95
3) Images from a Concave mirror Point a mirror at a distant bright object and see if you can focus an image on a piece of paper. (Hold the paper off to the side so you don t block the mirror with your paper). You will need to move the paper in and out until you find and focus the image. Notice that the image forms at a specific distance from the mirror if the screen is too close or too far away from the mirror, the image is fuzzy or it doesn t exist at all. Note that here the image is located in front of the mirror, and it projects on a screen, so this is a "real" image. a) How far is the sharp image from the mirror? This is the focal length of the mirror. b) Is the image upside down or right side up? c) Now find the big concave mirror and put your face into it scary? This is a magnifying mirror or shaving mirror. You have to be close to the mirror to see your face upright. Move away slowly. Keep moving away, until about ten feet or more. Summarize what happens along the way. 4) Assortment of lenses. Play with them, look through them, get a feel for what they do and try to use them to produce an image on a screen. Notice that the greater the curvature of the surfaces, the more they bend the light. Pick 8 lenses from the box and fill in the table with their characteristics. Lens Number Shape (concave or convex) Can it make an image on a screen? Does it work as a magnifying glass? Focal Length? Do you see any patterns? What shape(s) of lens can make a real image (one on a screen)? 5) Ghost Light Bulb Observe the image of the light bulb floating in the space above the light socket. a) Is the image real or virtual? How do you know? b) Is the image right side up or inverted? (be careful look at the object before you answer this question) c) Is the image bigger than, smaller than or the same size as the object? 96
6) Image from a Plane mirror Look at your image in the plane mirror. Ask your partner to stand behind the mirror, off to the side, so that it looks like he or she is standing right next to your image. e) Measure the distance from you to the mirror, and from the mirror to the partner. Is your image as far behind the mirror as the object (you), is in front? f) Measure the height of your image in the following way: Have your partner (who is still standing next to your image) hold a 2 meter stick and point their finger at the top of your image s head. How does the height of your image compare with the height of you, the object? g) The image-to-object ratio is called the "magnification," so if the image and object are the same size, the magnification is 1. If the image is twice as big as the object, the magnification is 2. What is the magnification in this case? Note that the image is located behind the mirror. You can see it, but you can not project it on a screen. This kind of image is called a "virtual" image. 7) Images in the eye. a) We have two eye models. Illuminate your hand with a desk lamp and place it about 50 cm from the two eye models. Find the image of your hand on the retina (the screen) at the back of each eye model. Is the image upright or inverted? If the image is inverted, why does the world not look upside down to us? When the image is clear in Eye #1, how does it look on the retina of Eye #2? b) Use the neon OPEN sign to make images on the retinas of both eye models. Which eye model does a better job of focusing on this distant object? Would you call this eye nearsighted or farsighted? c) Now use the ray box as the object, and place it about 50 cm from the eye models. How could you improve the focus of Eye #2 on nearby objects by using glasses? Would you use converging lenses or diverging lenses? Try using both kinds of glasses. Which lenses are helpful for this eye? Cool, huh? 97
8) Telescope putting lenses and mirrors together In stations 4 and 5, you saw that you could use a mirror or a lens to make an image of a distant object, but the images were pretty small. You also saw that you could use a lens as a magnifying glass, but it only worked when it was close to the object. Here you ll see how you can combine these two to make a telescope, which makes a magnified image of a distant object. Find the telescope s concave mirror and the image that it forms on the screen. (The little square mirror on the track is used to send the light off to the side, so that we can put the screen in a more convenient location.) You might need to move the screen slightly to get a nice focused image. Is this a real image or a virtual image? Now use the lens as a magnifying glass to magnify the image on the screen. When you have the image focused, take away the screen and you should see the image more clearly. Is the image upright or inverted? Try moving the eyepiece (magnifying glass) forward and back slightly to focus the telescope at different distances. To change the telescope s focus to a nearer object, which way do you have to move the eyepiece, toward or away from the telescope? Above is a picture of a real Newtonian Telescope (the same kind as our model). Before you leave, please put the screen back in place for the next person. 9) Optical Illusion Banks With your lab partners, observe each of the two optical illusion banks. The optical illusions are created using mirrors or lenses. Discuss with your lab partners how each illusion might be created, and write (or draw) your ideas below. Then check with the instructor for to see if your models are correct. #1 #2 10) Bending Light Total Internal Reflection Observe the laser beam as it strikes the interface of water and air or plastic and air. Change the angle gradually and see what happens. a) Sketch what happens when the beam comes through the water and hits the air at a steep angle. Show all reflected and refracted rays. b) Sketch what happens when the light comes through the water and hits the air at a grazing angle. c) At what angle does the light first become totally reflected from the surface (none escapes)? d) Shine the laser into the various light pipes. Notice that light beams are always straight until they reflect at a surface. Explain (with a sketch) how a fiber optic is able to guide the laser light through the bends. 98