Reflection and Refraction of Light In this laboratory, we are going to examine the reflection of light from surfaces, as well as the bending, or refraction, of light as it passes from one type of material into another. The theory behind this experiment will be discussed in each part of the writeup. Procedure: Navigate to: http://phet.colorado.edu/en/simulation/bending-light and click the play button. A window will pop up. Click on the "More Tools" icon, so the screen looks like this: File Edit View History Bookmarks Tools Help Bending Light 11.4 * + @ A https://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html C Q Search Ray O Wave 650 nm Material Air Index of Refraction (n) 1.000 Air Water Glass Material Glass Index of Refraction (n) |1.500 Air Water Glass -0- Normal Angles 4 Bending Light PHET. = Maximize the size of the window. This will make it easier for you to make the readings. Make sure that "Normal" is ticked below the toolbox (lower left). This turns on a dotted line perpendicular (i.e. normal) to the surface. We are going to measure the angle from the dashed line using the protractor. Click the red button on the laser to turn the light beam on. Also, grab yourself a protractor from the toolbox. Pull the protractor over so that the vertical black tick in the center bar and the two "zero" readings are exactly lined up with the vertical dashed normal line, and the top of the center bar is aligned with the interface between thewhite "air" and mauve"glass" regions (top and bottom halves of the screen). It is quite important Part 1: Reflection that you get this lined up properly. 1. Make sure that you have the material at the top selected as "Air" and the material at the You will see an image that looks like the image below: Note that we have labeled the incident bottom selected as "Glass". (incoming) beam, the faint reflected beam and the refracted beam. The reflected beam is faint because glass typically only reflects a small portion of the light incident on it, with most of the 2. Click and drag the laser to the incident angles shown in table 1 and measure and record the incident light passing through. In fact, the intensity of the reflected and refracted beams together reflected angle. What is the relationship between the angle of incidence and the angle of add up to the intensity of the incident beam. (If you like, you can check this using the green reflection? intensity meter in your toolbox). In the first part of this lab we will be concentrating on the reflected beam. In the second part we will be looking at the refracted beam. Table 1: Angle of Reflection, steps 2 and 3 Angle of Incidence Angle of Reflection Angle of Reflection (air (reflecting from glass) (reflecting from water) -reflected beam Trial 1 20 Trial 2 30 20 Material: Air Trial 3 60 incident beam Trial 4 750 Index of Refraction (n): 1.000 60 60 U Water Glass 3. Now change the material in the bottom section to water and repeat step 2. Does the angle of the reflected light depend on what surface the light is reflecting from? Material: Glass Index of Refraction (n): 1.500 4. At the top left, you will see a slider bar that allows you to select the wavelength of light coming from the laser. Currently it should be set at 650 nm for red light. Click the "Angles" Air Water Glass box at the bottom left. This will put exact readings of the angles on the protractor. Pull the laser so the reflected beam is not hidden by any grey boxes. Watching the reflected beam carefully, slide the slider on the laser to change the wavelength through the entire visible range. Does the ~refracted beam angle of reflection depend on the wavelength of the incident light? Let's review how to use the protractor. We will be measuring the "angle of incidence"- the angle Part 2: Refraction and Snell's Law between the incident beam and the normal, the "angle of reflection"- the angle between the reflected beam and the normal, and the "angle of refraction"-the angle between the refracted Light travelling through a vacuum travels with a speed c=3.00x10 m/s. When it enters any beam and the normal. The first step to get the angle correct is to align the protractor exactly as medium, it slows down, with its new speed given by v=c, where n>1 is the refractive index of described above. the material. Different materials affect light speed by different amounts, resulting in varying values for their index of refraction, as shown in Table 2 below for various materials. Now let's look at an enlargement of the protractor. In this case we can see that the angle Table 2: Index of Refraction for Materials 30 30 of incidence is where the red incident beam hits Vacuum 1.00 the scale on the protractor, and it reads 450. The Water 1.33 angle of reflection is where the faint reflected 60 60 Ethyl Alcohol 1.36 beam hits the scale on the protractor: again 45. Human Cornea 1.40 The angle of refraction is where the lower, beam Plexiglas 1.51 -90 90 hits the scale of the protractor in the blue region, Crown Glass 1.52 and it reads about 28.50. Make sure you are + Sodium Chloride 1.53 comfortable with making these measurements. Cubic Zirconia 2.15 60 60 Diamond 2.41 Now we are ready to get started. 2 3Snell's Law states : nl sinsl =nzsinl91where 01 is the angle of incidence, 02 is the angle of refraction, and n; and n2 are the respective indices of refraction of the materials This means that light passing -om a material with a low index of refraction (air) to a high index of refraction (glass, water, plastic) will be bent toward the normal. Light passing from a high index of retraction to a low index of refraction will be bent away 'om the normal. In this part of the experiment, we will use Snell's Law to measure the index of refraction for several materials. I. Make sure that you have the material at the top selected as \"Air" and the material at the bottom selected as \"Glass". Set the laser back to a wavelength of 650nm. 2. Click and drag the laser to the incident angles shown in Table 3 and measure and record the re'acted angle (the bottom part of the protractor). Get the most accurate reading that you can. 3. Given that the index of refraction for air, n1=1.000, rearrange Snell's Law to solve for H2. Using this formula, calculate the index of refraction for glass for each of the three trials in Table 3. Compare the average of your three values to that shown on the screen for glass. Did Snell's law work? Table 3: Angle of Refraction, steps 2 and 3 Angle of Incidence Angle of Refraction (in n; for glass (air) glass) Trial 1 30" Trial 2 45" Trial 3 60 average n 4. Now change the material in the bottom section to water and repeat steps 2 and 3 lling in Table 4. How does your average n compare to that shown on the screen? Table 4: Angle of Refraction, Step 4 Angle of Incidence Angle ofRe'action (in 112 for water (air) water) Trial 1 30" Trial 2 45 Trial 3 60 average n 5. Now change the material in the bottom section to \"Mystery A\" and use a couple of di'erent incident angles to give you the index of refraction for that material. Based on Table 2, what do you think this material might be? 6. Now change the material in the bottom section to \"Mystery B\" and use a couple of different incident angles to give you the index of refraction for that material Based on Table 2, what do you think this material might be? 7. With the \"Angles\" tick box selected so that the refracted angle is shown on the protractor, watch the refracted beam carefully, sliding the slider on the laser to change the wavelength through the entire visible range. Does the angle of refraction depend on the wavelength of the incident light? Which bends more, red light or blue light? What does this mean about the index of refraction? What does this mean about the speed of light? (You can drag the brown speed meter onto the beam to check its speed in any medium) Part 3: Total Internal Reection Ifa ray of light travels oor a medium of greater index of re'action to a medium of lesser index of retraction, the beam is bent away from the normal. If the angle of incidence is exactly the critical angle, the angle of the refracted ray is 90 degrees (i.e. it lies in the surface of the material. In this case, using Snell's Law, risingc =(l)sin(90)assuming the medium of lesser index of refraction (to which the light is going) is air with 112 = l and the medium of greater index of refraction (from which the light is coming) is the n|=n. In this case, we can solve for the critical angle using 1 sinl9c = n Ifthe beam is incident with an angle greater than the critical angle there is no refracted ray, and total internal reection occurs. 1. Set the laser wavelength back to 650 nm. 2. Make the material in the top part glass, and the material in the bottom part air. Now we will have the light going om a material with a high index (n = 1.50) to a low index (n=1.00). In this case, we expect the critical angle to be sin 9, = = 41.8\" 3. Move the laser upward so that the angle of incidence is about 10", and slowly pull it downwards, increasing the angle of incidence. Watch the refracted beam in the bottom part of the screen, As you increase the angle of incidence, it will slowly bend up toward the interface between the glass and the air. When the angle of incidence has reached the critical angle, you should see that the refracted beam is parallel to the interface (horizontal) and then disappears. At this instant, the intensity of the reected beam increases dramatically, since now none of the beam passes through into the air. Repeat this a couple of times to get an accurate estimate of your critical angle. Remember that you can read the angle of incidence on the zoomed in protractor using the fact that the angle of reection is the same as the angle of incidence. Record it in Table 5. 4. Repeat step 3 three more times with the upper material set to water, Mystery A, and Mystery B, and record the critical angles in Table 5. 5. Calculate the experimental values for n ti'om your experimental critical angles, and compare to the index of reaction you found for each material in part 2. Is there good agreement? Table 5: Determination of Critical Angles File Edit View Hatory Bookmarks Tools Help Bending Light 124 * + +( http/phet.colorado.edu'sims/html/bending-light/latest/bending-light. em.html c Q Search Material Critical Angle nexp n from Part 2 % difference Glass Environment Air ndex of Refraction (n) Water Mystery A Mystery B Part 4: Dispersion 650 nm In the final part of this lab, we will use the variation in index of refraction with wavelength to show that white light is composed of all colors of visible light. We will do this using a principle called "Dispersion" or separation of light. 1. At the top of the screen, click the "Prisms" icon at the center bottom. You will see a screen as shown below. Make sure the environment is set to "Air", the Objects are set to Objects Glass 'Glass" and that you have "One color" and "Single Ray" selected for the light, as shown Index of Refraction (n Refactions below. AALOO Water Glass Normal -0- Protractor O file Edit View History Bookmarks Tools Help Bending Light 1 14 Bending Light A https w/html/bending-light/latest/bending c Q Search PhET = Environment Air 3. Slide the wavelength selector for the light color from red to blue, watching the beam Index of Refraction (n) Air Water Glass exiting the right side closely. Does the position seem to depend on wavelength? (Hint: watch where the bottom of the beam intersects words in the bottom grey box). If you like, you can turn on the normal lines and grab a protractor to measure the actual angles, but you will need to rotate the protractor to align with the interface to get accurate readings. 650 nm 4. Which color of light bends more, red or blue? Based on this, predict what order the colors would be in a rainbow created by this prism, from top to bottom. (ROYGBIV or VIBGYOR)? 5. Now click on the selector shown here, center right, to switch to white light. You should see the refracted light separate out into a rainbow. Was your prediction in part 4 correct? AALOO Objects Glass 6. Rotate the prism around slowly, pulling the bronze knob upward to make the exiting Reflections Index of Refraction (n) beam move closer to the side of the prism. Does the amount of separation of the light Air Water Glass 7 Normal Protractor depend on the angle at which the prism is rotated? Why or why not? -0- 7. As you rotate the prism, you will see that at some point the exiting beam is reflected inside the prism, and comes out of the bottom of the prism rather than the right side. Bending Light PhET = Why is this? (Hint, look back to part 3)! Which color vanishes last? 2. Turn on the laser by hitting the red button. Then drag the triangular prism up into the This last experiment is very flexible. Pull in different shapes, experiment with turning on path of the laser beam. You can move it around on the screen by clicking and dragging. multiple rays, and showing the reflected beams. Change the material of the objects and the You can spin it by clicking and dragging on the yellow knob on the corner. Rotate it environment. If you make the prism out of air in a glass environment, what happens to your slightly, so that your picture looks approximately as below. rainbow