Name (printed) ACTIVITY A FIRST LOOK AT REFRACTION OBJECTIVE To observe optical refraction and theorize the mechanism for the observation. PROCEDURE 1. Start with no water in the cup and orient your line of sight so that the penny is just out of view. 2. While maintaining your position, slowly pour water into the cup. You should notice the penny come into view. QUESTIONS 1. What changes did you notice in the image of the penny after the water was added? The refracted image of the stem shown in this photograph (taken by Lindsay Yellen, Class of 2008) is in a remarkably different position than the actual stem. How is this similar to what you see in this activity? 2. In the space below, draw the path you believe the light took from the penny to your eye. Before adding water After adding water 1
INTRODUCTION As it turns out, there is a very predictable pattern that light follows when it moves from transparent substance to transparent substance. You know a bit about the bending from the diagrams on the previous page. Your job in this lab is to try to figure out the specific pattern that is to figure out what direction a ray of light will take as it enters a transparent substance if it approaches at a specific angle. To do that you ll begin by mapping out the paths that light takes as it passes from air to water at various angles of incidence. A pattern will begin to emerge. Look carefully for this pattern. LAB OPTICAL REFRACTION (PART 1) In Figure 1 below, three pins map out the path that light takes as it passes through the air and then into water (held in a semi-circular plastic dish). Notice that the light does not follow a straight path. (The light does not bend after moving from the water to the air because it hits this boundary at 90.) In Figure 2 Amelia illustrates how to find this path. She places two pins in front of the plastic tray to mark the path of the incident ray. Then she looks through the rear, semi-circular part of the tray and places the third pin so that it lines up with the first two pins when they are viewed beneath the surface of the water (see Figure 3). Normal Angle of incidence Angle of refraction Figure 2: She places the third pin so that it lines up with the first two pins when viewed beneath the surface of the water. Figure 1: Top view of set up Figure 3: Note all three pins are aligned when viewed along a path under the surface of the water. 2
PURPOSE To qualitatively observe optical refraction. To quantitatively measure optical refraction. To discover patterns of refractive behavior unique to a particular transparent substance. PROCEDURE 1. Fill the plastic container half full with water and put a piece of cardboard under the graph paper on the following page. 2. Place the container on the graph paper so that the straight edge is along the horizontal line at the middle of the page and centered. The curved surface should be facing you. Place a pin at the center point of the page. It is very important to check this placement frequently throughout the lab (Figure 1). 3. Place a second pin on the first incident ray a few centimeters from the container s flat side. 4. Looking through the water in the box from the curved side, turn the cardboard until the two pins are in line. 5. Place a third pin on this line of sight. When you look through the water (be certain your line of sight is under the surface of the water), the three pins will appear to be in a straight line (Figure 3). 6. Use the placement of the third pin to measure the refracted angle. Measure and record this to the nearest 0.1. 7. Repeat this for all incident rays. 8. Make calculations necessary to complete the data table. For all calculations, record to two places past the decimal. DATA Record all data and calculations to three significant figures Angle number Incident angle, i Refracted angle, r ( i) 2 r i r sin i sin r cos i ( tan r) 2 tan i cos r 1 20.0 2 25.0 3 30.0 4 35.0 5 40.0 6 45.0 7 50.0 3
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QUESTIONS/CALCULATIONS 1. Which column seems to have the most interesting pattern? What is that pattern? 2. If you could do this lab with NO experimental error, what would you expect to see in the column that has the most interesting pattern? Be specific. 3. If you did this lab with another transparent substance, instead of water, what would you expect to see in the column that has the most interesting pattern? Be specific. (Hint: What would be the same and what would be different? GET CHECKED BEFORE MOVING ON PROCEDURE OPTICAL REFRACTION (PART 2) 1. Get a new plastic container from me and add Wesson oil to it. 2. Use the technique you just learned in order to determine the quantitative degree of refraction for Wesson oil. Use a procedure similar to the one you used in Part 1. However, you can choose however many measurements you think are necessary. Consider your responses above when deciding which calculations should be done. 3. Make a neat, well-labeled table for your data in the space below. 4. Make and show your calculation for the number that represents the way light refracts in Wesson oil. 5. Return the oil to the Wesson oil container. 6 GET CHECKED BEFORE MOVING ON
ACTIVITY OPTICAL REFRACTION The photo to the right (taken by Victoria Palmer, Class of 2011) shows a glass with five different liquids layered on top of each other (alcohol, oil, water, dish soap, and honey). Their different indices of refraction cause the inserted stick to appear broken. You should be able to show this phenomenon using Snell s Law in the diagram below. Use a protractor to measure the incident angle shown below. Then calculate the path the light would take as it passes through each of the following layers of material and then back out into the air. Show the path carefully, label all angles, and show all of your calculations to the left of the diagram. Important: Don't round numbers to be used in further calculations Calculations i Normal Air Diamond Water Flint glass Ethyl alcohol Substance Index of Refraction Air 1.00 Water 1.33 Ethanol 1.36 Fused quartz 1.46 Crown glass 1.52 Flint glass 1.63 Diamond 2.417 7 Air GET CHECKED BEFORE MOVING ON
REFRACTION QUESTIONS AND PROBLEMS 1. The refracted light leaving this vase produces two images of the spoon inside. Assume that in the line drawing that you are looking from the top of the vase down. Refract the rays drawn in the diagram to show how the two images are produced. (You don t need to do calculations here. Simply bend the refracted rays in the proper direction toward or away from the normal shown.) Trace the refracted rays back into the vase as dashed lines in order to show the two images produced. Top view 2. In the Refracted Images demonstration, a volunteer viewed an object through a set of prism-shaped glasses. The volunteer wasn t able to grab the object because, although he believed he knew where the object was, he was actually looking at a refracted image of the object. In the diagram below, there is an object, a ray of light coming from the object and one of the prism-shaped eyepieces. Show the path the light takes as it moves through the prism and then into the eye and then show how that leads to the formation of the refracted image. Object Light Ray prism 8 GET CHECKED BEFORE MOVING ON
For credit, you must provide a clear and complete explanation for each of the remaining questions. 3. A swimmer (in a lake whose surface is perfectly calm) looks upward through the water at the setting sun. His view of the sun will make it appear... a. lower than it really is c. the same as viewed above water b. higher than it really is d. smaller than it really is Provide a drawing for your explanation 4. If you wish to spear a fish with a regular spear, you should compensate for refraction between the air and water and throw your spear a. directly at the sighted fish. b. above the sighted fish. c. below the sighted fish. Provide a drawing for your explanation 5. The figure below shows the path of a portion of a ray of light as it passes through three different materials. The figure is drawn to scale. What can be concluded concerning the refractive indices of these three materials? a. n 1 < n 2 < n 3 c. n 3 < n 1 < n 2 b. n 3 < n 2 < n 1 d. n 2 < n 1 < n 3 n 1 n 2 n 3 6. A ray of light, initially traveling in a vacuum, is incident on the surface of a flat transparent material as shown in the diagram below. Part of the light is reflected at the surface and part is refracted. The angle between the reflected ray and the refracted ray is a. less than 40 d. between 100 and 140 b. between 40 and 50 e. more than 140 c. between 50 and 100 40 O Vacuum Transparent material 7. The speed of light in crown glass is 2 x 10 8 m/s. The index of refraction of crown glass is: a. 1.0 b. 1.5 c. 2.0 d. 3.0 8. The speed of light in a transparent gel is 2.20 x 10 8 m/s. If the angle of incidence of a beam of light directed at the surface of the gel is 30 what is the angle of refraction? a. 1.36 b. 21.5 c. 40.9 d. 47.2 9