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Transcript
Reflection and Refraction Lab MSED 250 Purpose: To explore light interaction with planar and nonplanar mirrors and lenses. Materials: Power supply Light box optical set Meter stick optic bench Optic bench light Optic bench Procedure: 1. The following diagram depicts a ray of light striking the surface of a plane mirror. Describe the path that you expect the reflected light beam to take. 2. Place the 4-slit gate in the holder and observe how the light beams reflect off the mirror. If you were to replace the plane mirror in front of the beams with a white piece of paper, like an index card, would the paper also reflect light? Write down your thoughts. a. Place a white card in place of the mirror. Now take a second sheet of paper and hold it in front of the card on which the light beams are striking. Try to catch any reflection off the first card with the second. Can you see any evidence that the light lines from the first card are being reflected on the second? b. If you can or cannot see evidence, provide a reason why this may be so? 1 3. The 4-slit gate effectively makes parallel light beams. The light beams look like this: a. How could you use plane mirrors to make the individual light beams all meet at one point? b. Would this also work if, instead of four separate beams, you had one broad beam? c. Another way to do this is to use a curved mirror. Set up the light box with the four slit gate. There are two curved mirrors in the light box kit. A concave mirror can also be used in place of plane mirrors to bring light beams to a common focus. Determine the focal distance (the distance from the mirror to the focus point) for both mirrors. i. Highly curved mirror Focal distance = _____ cm. ii. Lesser curved mirror Focal distance = _____ cm. Refraction Light bends when it passes from one medium to another. 4. Examining refraction a. Set up the light box with the narrow single slit gate b. Place the Lucite rectangle in front of the light beam with the light entering through one of the longer sides. c. Rotate the rectangle in both directions i. In what position must the rectangle be in order to see no light bending? ii. Where does light bending occur? iii. Note what happens as you rotate the rectangle away from this position: 2 1. Does the angle of refraction stay the same? Explain. 2. Does the intensity (brightness) of the refracted beam stay the same? Explain. 3. Does the width of the refracted beam stay the same? Explain. d. Replace the rectangle with the Lucite triangle. i. Let the light beam enter the triangle perpendicular to one of the faces, but not centered on the face. 1. What do you observe? Remember our conversations regarding diamonds. Can you see reflection and internal reflection? 2. Slowly rotate the triangle. At some point the white light will be bent to the point that the white light is separated into its spectral colors. a. Which color gets bent most? b. Which color gets bent least? c. Which color is on the outside of a rainbow? d. Which shape, the rectangle, the equilateral triangle, or the 30-60-90 triangle provides the greatest separation of color? Why do you think this is so? e. Is there any difference in the “quality” of the rainbow if you replace the narrow single slit with the wider one? How about if no gate is present? e. Replace the single light gate with the 3-slit gate and the triangle with the half circle, with the flat side facing and perpendicular to the light gate. i. What do you observe? 3 ii. What happens if you rotate the half circle to the right or left? iii. Predict what you will see if you were to turn the half circle completely around so that the light struck the curved surface first. Write your prediction and then try it. iv. Explain the results v. What two factors are necessary for a light beam to refract? Gemstone Refraction Index Determination One of the most important tools in identifying gemstones is the refractometer. This tool allows you to determine the refractive index of the polished surfaces of gemstones. A high R.I. liquid is used to make an airless contact between the stone and a piece of calcite in the refractometer. The stone is moved along the calcite until a green band is seen. Check out the demonstration stations to see if you can identify the two gemstones. Gemstone 1: Gemstone 2: Lenses Recreate the pinhole device from the previous lab using the meter stick optic bench, lamp, screen, and opaque card. 1) Speculate what would the image on the screen would be like if you placed a biconvex lens in place of the pinhole. 2) Configure the optic bench as noted above. (You may need to adjust the distance from the lens to the screen.) Alternate between the pinhole and the lens. Note your observations here. Can you explain your observations? How do the results compare? Which of the illustrations below best describes how light acts when it travels through the lens, or are they all accurate, or are they all wrong? You can check your thoughts here. 4 3) Use the lens and arrange the apparatus so the screen displays a sharp upside down image that is the same size as the bulb. Keep the lens in place, but move the screen, first closer to the lens, then farther from it. Describe what happens. 4) Replace the lens with the pinhole and repeat the movement of the screen. Compare the results with those discovered when you used the lens. 5) Put the lens back in place and adjust the screen so the image is in sharp focus. i. What do you think will happen to the image on the screen if you use a card to cover the top half of the lens? Write your prediction below. ii. Perform the experiment and note your observations. Do they agree with your prediction? Can you explain what you see? iii. Would it make any difference if you covered the center of the lens and left the outer parts clear? 5