Physics 2B Spring 2023 (Lab 7)

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San Jose State University *

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Physics

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Apr 3, 2024

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7 - Reflection and Refraction, Parts 1 and 2 CAUTION! This lab uses lasers. All lasers are potentially dangerous. Never look directly into the beam, or point a laser beam at another person. Always think about where the laser beams are going if they leave your immediate area and be aware of reflected laser beams; it is highly recommended to block them from leaving in the first place using the wooden beamstops. IMPORTANT: DO NOT LOWER YOUR HEAD TO THE LEVEL OF THE LASER TO ALIGN IT. BACKGROUND Today we study the two basic relations; the law of reflection, and the law of refraction (known as Snell's law). The Law of Reflection The angle of incidence = the angle of reflection. These angles are measured from the normal, a line perpendicular to the reflecting surface, as shown by the dotted line in Figure 1. Here the incident light is coming in from the upper left, it strikes the surface at some angle, then reflects off the surface at the same angle. Figure 1. Illustration of the Law of Reflection Snell's law Snell's law is a relationship that states how light refracts when passing from one medium to another. Again, the angles of incidence and refraction are measured relative
to the normal line. If n 1 is the index of refraction of medium 1, and n 2 is the index of refraction of medium 2, then Snell's law is: n 1 sin(ɸ 1 ) = n 2 sin(ɸ 2 ) Notice there is also a reflected ray at the surface; the law of reflection tells you it will be ɸ 1, the same angle as the incident ray. Part 1: Testing The Law of Reflection / Experimental Design Materials: Laser, Triangular Mirror, PASCO protractor (The PASCO protractor is the white circular platform with angle measurements.) 1. Place the flat side of the triangular mirror along the line on the PASCO protractor labeled “Component”. 2. Before turning on your laser source, set it up so that it is pointing close to the “normal” of your mirror (0 o on the protractor). Remember not to lower your head to the level of the laser at any point in this lab; look down from above. 3. Turn on your laser (single laser beam mode), and align your mirror so that the laser beam is as close to “normal” (0 o ) as you can make it. What do you observe to be special about this angle? Does your observation make sense? Explain.
4. Design an experiment to test the Law of Reflection (see Figure 1 above). You must have at least 4 separate measurements. Explain your experimental plan in the space below. 5. Use “Insert Table” to design a data table that will work well for your experimental design. It must be clear and understandable by anyone who reads the table. When taking your measurements, estimate how precisely you can measure an angle, this will be your measurement error. State this measurement error is in your table, or record it near your table. Insert Data Table Here, then add measured values 6. Decide how to analyze your data to test the validity of the Law of Reflection. Explain and perform your analysis here. 7. To what accuracy (by how many degrees did your values for the angles of incidence differ from the angles of reflection) does the Law of Reflection seem to hold? Is this better, worse, or about the same as your estimated measurement error noted in step 5?
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Part 2: Index of Refraction Measurements Materials: Laser, PASCO protractor, Semicircular Glass, Semicircular Water Dish (Filled) 1) Replace the triangular mirror with the semicircular glass lens as shown in the above diagram, with its flat edge along the “component” line. 2) The center of the glass should be on the normal line of the protractor as shown by the dotted line in the diagram. 3) When shining the laser beam on the flat side of the glass the refracted ray should enter the glass at the center of the protractor. If aligned correctly the laser beam will change direction (bend) once it enters the glass, but not when it exits the glass. Ask your instructor if you are not sure about the alignment. 4) Complete the experiments that allow you to fill out the data table below. See Figure 2 and Figure 4 for reference. Refraction Data Table for Glass Incident angle ( ɸ 1 ) In air Refracted Angle ( ɸ 2 ) In the glass Calculated: sin( ɸ 1 )/sin( ɸ 2 )
15 o 30 o 45 o 60 o 75 o 5) Explain why the average value of the final column should equal the index of refraction of the glass. (The index of refraction of air is 1.00.) 6) Show the percent error calculation for your average measurement of n glass . (You can assume that n glass = 1.5.) 7) Repeat the same experiment for the semicircular container of water. Carefully follow the alignment steps 2) and 3). Refraction Data Table for Water Incident angle ( ɸ 1 ) Refracted Angle ( ɸ 2 ) Calculated: sin( ɸ 1 )/sin( ɸ 2 ) 15 o 30 o 45 o 60 o
75 o 8) Show the percent error calculation for your average measurement of n Water = Average of Calculated: sin( ɸ 1 )/sin( ɸ 2 ) , where the known value is n Water =1.33. 9) Instead of analyzing each data point on its own, as you did in your data tables, it is better to look at all the data at once on a graph and use a curve fit. Go back to your refraction experiment for glass and make a graph of sin ( ɸ 1 ) vs. sin ( ɸ 2 ) . Note that this graph will be of the form y=mx, therefore use your graphing software to display the trendline and the slope of this trendline. Attach the labeled graph below. graph 10) How did you determine the index of refraction of glass from this graph? Based on Snell’s law, state why this is reasonable.
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