Unit 8: Light and Optics

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1 Objectives Unit 8: Light and Optics Explain why we see colors as combinations of three primary colors. Explain the dispersion of light by a prism. Understand how lenses and mirrors work. Explain thermal radiation. Identify evidence that light is a wave. Overview What is light? How is it created? How does it travel? What is the difference between white light and colored light? How does a prism create a rainbow from white light? How does a mirror or a lens form an image of an object? Because vision is such an important way that we learn about our surroundings, the study of light has long been a prominent part of physics. As always, this week in class we will only scratch the surface of this vast topic. Set-up The physics department actually has a fair amount of optics equipment, enough almost for each group to have all the equipment necessary for each activity. There will be a few activities for which it will be necessary to combine supplies with other groups. The most difficult part will be finding enough electrical power outlets. Because we will be studying light, we will keep the room lights off. To read or work on your work sheets, you may need to leave the room for somewhere with more light, and then come back in. This is an inconvenience necessary for being able to do the activities at all. Equipment Each group should obtain: light source and power adapter (see diagram) optics box glass block and prism mounted concave lens loose convex lenses, thin and thick lens holder diffraction gratings, 1 per person PHYS

2 Procedures The activities may be done in any order. However, Activity 4 will make more sense if it is done after Activity 3. Activities 1 and 2 use equipment that is not part of every group s supplies, so you should do them whenever the equipment becomes available. There are several worksheets accompanying this lab activity. As always, each person in each group should do each worksheet. You won t learn from what you don t do. Clean-up At the conclusion of the class period, please return all supplies to their proper places on the cart. Activity 0: Spectroscope Purpose A spectroscope allows you to see light separated into its different wavelengths. You will begin by making yourself a spectroscope to use for the rest of the unit. Equipment cardboard or paper tube, aluminum foil, tape, razor blade, slide-mounted diffraction grating Procedure Cut a piece of aluminum foil large enough to fit with overlap over an end of the tube. In the center of the foil, make a straight, clean razor cut about 1.5 cm long. Place the foil piece over the end of the tube, centering the slit in the opening. Fold the edges of the foil over the tube and tape it down to make a light-tight seal. To observe the different wavelengths emitted by a light source, place a diffraction grating over the open end of the tube and hold the grating end of the tube against your eye. Rotate the tube so that the slit is vertical. Look at the light source through the slit; spectra of the light will appear to the sides of the slit. Use this spectroscope in the following activities that call for it. If you don t like how it works or the slit of your spectrometer becomes damaged, make another! PHYS

3 Activity 1: The different colors of light Purpose What is light? What is color? Additional supplies slit foil tube spectroscope, gas discharge tubes, short metal bar, colored pencils Overview You will observe the components of light from a variety of sources. Using your spectroscope, you will separate the different colors of light from each source, and compare and contrast the different mixtures. Procedure Incandescent light 1. Turn on your light source so that you are facing one of its white light beams, or use a bare vertical-filament incandescent light. Look at the white light through the slit of your spectroscope. Describe and sketch the spectrum you see. Gas discharge light 2. Look through your spectroscope at the light from the different gas discharge tubes. Sketch the patterns you see from each. PHYS

4 Sunlight 3. Find a patch of sunlight, either in the hallway or outside. Place the short metal bar in the sunlight against a dark background and position it so that the sunlight brightly glints off it. Look at the sunlight reflected from the bar through your spectroscope. (Or, if your foil slit is narrow enough and there are any clouds, look at a bright patch of a white cloud.) DON T look at the sun directly, even through your spectroscope! Describe the spectrum you see. 4. Look very carefully at the spectrum. It should contain some thin dark lines. Where are they? Describe their position and appearance. Clean up At the conclusion of this activity, return your metal bars to the cart. Questions to consider Compare and contrast the light from the discharge tubes and the incandescent light source. Compare and contrast the light from the different gas discharge tubes. Compare and contrast the light from the sun and the incandescent light source. Compare and contrast the light from the sun and the gas discharge tubes. PHYS

5 Activity 2: Absorbing and combining colors Purpose What determines the colors we see? Do colored filters add color to white light, or do they take it away? Additional supplies Red, green, and blue light sources, color computer monitor, red cellophane, blue cellophane, printed color gradients. Overview You will observe the effect of putting a colored filter between a light source and your eye, and investigate the difference between white light and colored light. Procedure White light with cellophane filters 1. Look at one of the white lights from your light source through your spectroscope. Then place a red sheet of cellophane over the light and look at the transmitted light through your spectroscope. Compare and contrast the two spectra. 2. Remove the red cellophane and replace it with a sheet of blue cellophane. Look at the transmitted light through your spectroscope. Compare and contrast the spectra of the white light, the red light, and the blue light. PHYS

6 Colored light sources Slide the bottom filter of your light sources so that the red, green, and blue lights are transmitted. With foil, block the white-light openings of your light sources. Additionally, block off colors with foil so that the source emits only red, green, or blue light. Look at each color in turn with your spectroscope. Describe the spectra you see. red: green: blue: Color projectors This apparatus comprises three separate light sources projecting onto a screen. 1. Look at the region where the light from the red and green sources overlap. What color do you see there? Do the same for the other combinations of two of the three primary colors. red and green: red and blue: green and blue: 2. Now observe where all three sources illuminate the same location. What color do you see where all three overlap? Computer monitor white screen Turn on a computer monitor and bring up some white screen (perhaps create a new MS Word document). Place a thick convex lens in the holder, or use another hand magnifier, to magnify the screen so that you can see the individual pixels. (You need a high magnification to get a good view of the individual pixels. If you are using a single lens, you will probably get a decent image only in the very center.) In the space below, draw how the pixels are arranged and what their colors are. PHYS

7 Translucent color filters Hold a filter disk up to the light and look through it. Overlap two and three filters of different colors and look through both, or all three, together. What colors do the individual filters transmit? What colors do the combinations transmit? Does it matter which filter is in front and which is in back? Printed color gradients These are printed paper sheets. The numbers to the left and right of the color-gradient bars identify the densities of the cyan (C), magenta (M), and yellow (Y) inks making up the colors. The densities vary continuously across each bar. Under magnification, try to observe the changes in ink coverage from left to right. Can you see the individual dots of ink? What colors are they? Do the reported ink densities and observed colors make sense? Computer screen gradient bars Access the color gradient bars page from this unit on the class web site. This page shows color gradients on a black background. The top line shows the pure primary colors fading to black; the second line the secondaries fading to black, the third and fourth lines show the primaries and secondaries fading to white, the next three show the primaries fading to secondaries, and the last line shows white fading to black. Look at the bars through a magnifier to see how the pixel intensities vary across each bar. What pixel intensities create the different observed colors? Clean-up Return the cellophane filters and color gradient sheets and log off the computer. PHYS

8 Questions to consider What colors are needed to make light that appears white? When white light passes thorough a colored filter, does the transmitted light have color added to it or taken away from it? How does mixing colored inks create new colors? What color results from combining red and green light? What color results from combining red and blue light? What color results from combining green and blue light? What colors do color computer screens actually emit? What do cellophane filters do to convert white light to colored light? PHYS

9 Activity 3: Refraction with blocks and prisms Purpose In this activity you will explore the basic physics behind the bending of light by lenses. Overview When light passes through a transparent object, it does not just sail right through. Instead, light travels slower in all transparent objects than in empty space. Because light is a wave, this causes its path to bend when it passes obliquely from one transparent material to another. Procedure Glass block 1. Set up your light source so that light is emitted through one slit at the bottom. Shine the light at an angle onto a flat white surface to make a straight, thin beam. 2. Lay a blank piece of paper under the glass block. Shine the beam of light into the block at an angle, as before. Trace the outline of the block on the paper. Mark on the paper the path of the light beam into and out of the block. Change the angle of the light beam and again trace the beam into and out of the block. Trace the beams for a total of four different angles. Remove the block and connect the paths for the different angles you used. 3. How is the direction of the light beam coming out of the block related to its direction going in to the block? Trapezoidal prism 1. Place the trapezoidal prism from the ray optics kit frosted-side down on another blank piece of paper. Shine the beam of light into the prism into the pointed side of the long edge. Mark the path of the beam into and out of the prism on the paper. light 2. Reverse the direction of the light source so that the beam enters the prism along the path it previously came out. Along what path does it now come out of the prism? 3. Repeat the above steps with the light entering the block at slightly different angles. 4. How is the light beam s direction coming out of the prism related to its direction coming in to the prism? PHYS

10 Questions to consider When light passes from air into a glass block, how is the direction of its path inside the glass related to its direction in the air before it enters the glass? Light travels slower in glass than in air. How is the bending of light rays into glass related to the bending of ocean waves in shallow water? When light enters glass from air at an angle, along what direction does it travel inside the glass? When light enters air from glass at an angle, along what direction does the light travel in the air? Why doesn t a glass block change the direction of a light beam that passes through it? Why does a prism change the direction of a light beam that passes through it? PHYS

11 Activity 4: Refraction with lenses Purpose In this activity you will observe the bending of light by curved glass or plastic surfaces. Additional Supplies Lenses worksheet: Paul G. Hewitt, Conceptual Physics tenth edition Practicing Physics Workbook, pp One for each group member Overview Lenses, like prisms, change the direction of light that passes through them. Their curved surfaces allow them to make parallel light rays converge onto or diverge away from a focal point. This property enables them to form images. Procedure Answer questions 1 4 of the Lenses worksheet. Ray optics kit 1. Set up your light source to produce five parallel light rays. Place the source so that the rays fall at an angle onto a blank sheet of white paper, showing five parallel beams of light. Place the convex lens from your ray optics box frosted side down on the paper so that the light beams pass through it. Mark the outline of the lens on the paper. Also mark the paths of the five rays on the paper into and out of the lens. 2. Repeat the previous step with the concave lens. Round lenses 1. Look thorough a mounted concave lens. (You can identify a concave lens by placing it against an object and looking at the object through the lens as you move the lens away from the object: if the image gets smaller and smaller, it is a concave lens.) Even though I ve already given this answer away, describe how the image of an object changes as the lens moves away from the object. 2. Is the image upright or inverted compared to the object? 3. Now place a thin convex lens in the lens holder. Place the lens up against an object and look through the lens as you move it away from the object. Move the lens at least one meter. Describe how the image of the object changes as the lens moves away from the object. PHYS

12 4. Now work question 6 of the Lenses worksheet (skip question 5). It is easy to make false starts, so do it in pencil. See me for help if you are confused about what it asks you to do or how to get started. Questions to consider What happens to parallel light rays entering a convex lens? What happens to parallel light rays entering a concave lens? Which kind of lens can be used as a magnifying glass? Which kind of lens can be used in a slide projector? What does an image in a concave lens look like? Why does an image in a convex lens change so much as the lens-object distance varies? A convex lens is also called a converging lens. Why is it called that, and why does its shape create that optical effect? PHYS

13 Activity 5: Dispersion Purpose In this activity you will learn why a prism separates a beam of white light into colors. Overview Prisms bend light because light travels more slowly through glass and plastic than through air. However, all light is not alike! It turns out that different colors of light have different speeds in glass and other materials. This difference produces effects that are sometimes beautiful and sometimes annoying. Procedure Set up the light source so that it produces one beam of white light from a single slit. Place a glass or plastic prism in the beam so that the light passes through a corner and is broken into the different colors. 1. Which color bends the most from its original path (from the source)? 2. Which color bends the least? 3. Sketch below the path of the beam of light into and out of the prism, including the colors of the light. Questions to consider Which color of light travels the fastest through the prism? The slowest? Why does a prism break white light into colors? Why is the focal point of a lens slightly different for different colors of light? The difference between the speeds of light of different colors is comparatively very large in diamond. How does this property contribute to the fire of a diamond? PHYS

14 Activity 6: Diffraction and Interference Purpose Observe the wavelike diffraction and interference of laser light. Overview Particles travel in straight lines until they strike a solid object. Waves diffract around obstacles and interfere when they combine. Does light behave as particles or waves? We use laser light for this activity, because a laser is a convenient (now that lasers are widely available) source of coherent light. Materials Laser, clamp to hold laser, slide holder, interference slit slide, slide holder mount, projection screen Procedure FIRST, MAKE CERTAIN THAT NOBODY IS IN THE PATH OF THE LASER LIGHT! Although the laser has relatively low intensity, its pencil beam is more intense than light from an ordinary source. If it shines into someone s eye, it can cause permanent damage! Arrange the laser so that its beam falls onto the projection screen. Turn the laser on. Single slit 1. Place the interference slit slide in the way of the laser beam. Have the beam pass through a wide single slit and through a narrow single slit. Observe the projection of the beam on the screen in both cases. Sketch both patterns, identifying which is which. 2. Identify similarities and differences between the two patterns. PHYS

15 Double slit 1. Now, have the laser beam pass through a widely-spaced double slit and through a closely-spaced double slit. Observe the projection of the beam on the screen in both cases. Sketch both patterns, identifying which is which. 2. Identify similarities and differences between the two patterns. Questions to consider Does the single-slit pattern on the screen get narrower or broader as the slit becomes narrower? How did the water waves behave when they passed through a single wide slit and through a single narrow slit? Does the double-slit pattern on the screen have nodes and antinodes? What do node and antinode mean for light? How did the water waves behave when they passed through a double slit? What similarities and differences have you observed between how the laser beam and the water waves behave after passing through slits? Activity 7: Thermal radiation Purpose In this activity you will observe the relationship between the temperature of an object and the characteristics of the light it emits. Overview You will yet again observe the light emitted by an incandescent filament. By changing the electric power running through the filament, you change the filament s temperature. This affects both the intensity (quantity) and frequency (color) of the light it emits. PHYS

16 Materials vertical-filament incandescent bulb controlled by a variable power source, diffraction grating Procedure 1. Turn the dial of the power source to zero. Make sure the bulb is plugged in to the power source. Flip the toggle switch of the power source to the on position. Slowly dial up the power until the filament just begins to glow. What color does the filament glow? 2. Look at the glowing filament through a diffraction grating to see its spectrum. What does the spectrum look like? What colors are most prominent? 3. Dial up the power a bit more. How does the light s intensity change? Its apparent color? Its spectrum? 4. Dial up the power still more. (Please don t leave the dial at its highest setting for long; the bulb will quickly burn out at high power.) How have the light s intensity and apparent color changed? Observe the spectrum again. What are the spectrum s general features? Which colors are most prominent? Questions to consider How does the light s intensity change as the filament s temperature increases? How does the spectrum change as the filament s temperature increases? How do the relative intensities of the different colors change as the filament s temperature increases? What is the approximate relationship between temperature of the filament and the frequency of the light it emits? PHYS