Physics Lab (Online Simulation) Light Reflection & Refraction Critical Angle, Spectrum Electricity and Light TA name: Due Date: Student Name: Student ID: This lab uses the Remote lab platform from PhET Interactive Simulations at University of Colorado Boulder, under the CC-BY 4.0 license. Type all your answers in BLUE This pre-lab is worth 5 points. 1) What does each term in equation [1] represent? 2) What does the term Normal mean in the context of dealing with an optical surface? 3) Describe how light behaves in relation to the Normal when travelling between two interfaces in the following cases: From a less dense to a denser medium Physics Lab (Online Simulation) From a denser to a less dense medium 4) A convex lens is also known as a (converging, diverging ) lens and a concave lens is known as a (converging, diverging) lens. 5) Describe where the focal point for a convex and a concave lens located? Objectives: Upon successful completion of the laboratory exercise you will 1. Verify law of refraction 2. Find index of refraction of an unknown material 3. Create spectrum of light using prism 4. Find criticle angle required for total internal reflection Theory: Whenever a wave traveling in some medium encounters an interface or boundary with another medium either (or both) of the processes of (1) reflection and (2) refraction may occur if the speed of the wave is different in the two media. If the wave being considered is light, the speed of light in any medium is characterized by the index of refraction for the medium, n, where n  c/v [1] where c is the speed of light in a vacuum, and v is the speed of light in the medium. (Note that for a vacuum or air n=1.00.) Physics Lab (Online Simulation) Reflection In reflection, a ray of light traveling in a straight line in medium 1 encounters an interface with medium 2 and the incident ray is reflected (or bounced) backed into medium 1 at the interface of the two media as depicted in Figure 1. R I medium 2 medium 1 Normal Figure 1 The Normal is a reference line that is always perpendicular to the surface at the “point of impact” where the light is incident onto the surface. If the ray incident on the interface makes an angle I with the normal to the surface at the “point of impact” on the interface, the reflected ray will make an angle R with the normal from the “point of impact” equal to the incident angle I. The relationship between the angles I and R for all reflections is called the Law of reflection which can be stated in two parts: 1. I = R , and 2. I and R are coplanar, (i.e. lie in the same plane.) [2] For light traveling parallel to the principal axis and is incident on a spherical (or cylindrical) surface with a radius R, the reflected light will cross the principal axis at a focal point, f, from the vertex (see Figure 2) such that f = R/2 [3] Physics Lab (Online Simulation) where R is the radius of curvature. Normal f principal axis R radius of curvature Figure 2 Refraction In refraction, a ray of light traveling in a straight line in medium 1 encounters an interface with medium 2, penetrates the interface and then moves in a straight line in medium 2 as depicted in Figure 3. If the incident ray originally in medium 1 makes an angle 1 with the normal to the surface in medium 1, the refracted ray will make an angle 2 with the same normal in medium 2. The relationship between 1 and 2 for each refraction is called the law of refraction which can be stated in two parts: 1. n1 sin 1 = n2 sin 2 , and 2. 1 and 2 are coplanar. [4] 3 = 1 2 Normal 2 n1 medium 1 n2 n1 medium 1 Normal 1 medium 2 Figure 3 Figure 3 represents two interfaces the first is from a lower to higher index of refraction (medium 1 (n1) < medium 2 (n2)), and the second is from a higher to low index of refraction Physics Lab (Online Simulation) (medium 2 (n2) > medium 1 (n1)). At the interface of n1 < n2 the angle 2 will be less than 1 in relation to the normal which is in agreement with equation [4]. Likewise when n 2 > n1 the angle 3 will be greater than 2 in relation to the normal. From this it can be stated that: A light ray incident at an angle  from the normal will bend towards the normal when traveling from a less dense medium to a denser medium, and bend away from the normal when traveling from a denser medium to a less dense medium. Critical Angle For any interface where light travels from a dense to less dense medium there can be found an angle of incidence from the normal which will cause the light ray not to penetrate the interface but to travel along the interface as shown in Figure 4. Using Figure 3, it can be shown that if 2 is increased from the normal, 3 would eventually become 90 to the normal therefore the will travel along the surface of the interface. The angle 2 at this point is called the critical angle. From this it can be found that: lesser medium n 1 sin  c = n 2 sin 90, or 2 = 90 sin  c = denser c Normal medium n2  c = sin −1   n1 n2 [5] , or  n 1  Figure 4 In equation [5], (n2/n1) is called “the index of refraction of medium 2 relative to medium 1”. Note that if medium 2 is air then n2 = 1 and that n1 can be found by n1 = 1/ sin c [6] A phenomenon called total internal reflection occurs when the angle of incidence of the light ray from the normal at the dense to less dense interface exceeds the critical angle. At all angles where  > c all of the light is reflected back into the denser medium. Fiber optics is based upon this principal. Procedure: Physics Lab (Online Simulation) PhET Simulation “Bending Light” at https://phet.colorado.edu/en/simulation/bending-light Include Screen shots of each experiment in your results Procedure: Part A Setup 1. Open the simulation “Bending Light” at PhET. Use the address above. 2. Click on Intro. 3. Leave the default for entry material at “Air”. Choose “Glass” for the exit material (See pic on right). Record the index of refraction of glass into Table 1 4. Choose the protractor and set the laser to an angle of incidence, θ1, at 30°. Recall: angles are always measured from the Normal. 5. Ignore the reflected ray (the ray that remains in air). Using the protractor, measure the angle of refraction, θ2, of the laser and record in Table 1. 6. Repeat steps 4 and 5 of this lab for 4 more angles of incidence of your own choosing. Record the results in Table 1. Data: Note* For best results center the protractor where the beam splits on the screen Table 1 Trial n1 θ1 (degrees) θ2 (degrees) Sin θ1 Sin θ2 1 30 nglass 2 3 4 5 Observations and Calculations: 1. Classify the bending of light as exhibited by the ray diagrams. According to your data, is light refracted away from or toward the normal as it passes at an angle into a medium with a higher index of refraction? 2. Calculate sin θ1 and sin θ2 for each trial. Record the results in Table 1. 3. Calculate n2 (glass) for each trial. Record the results in Table 1. Physics Lab (Online Simulation) 4. Compare the values for index of refraction of glass for each trial (values in last column). Is there good agreement between them? Would you conclude that index of refraction is a constant for a given medium? 5. Compare your calculated n2 with the given index of refraction given in the lecture notes, nglass. Do they agree? Explain why it does or doesn’t. Calculate the percentage difference. Procedure: Part B Setup 1. Reset simulation and choose “Mystery A” for the refractive material. Set the incident material to air. 2. Choose the protractor and set the laser to an angle of incidence, θ1, at 30°. 3. Ignore the reflected ray (the ray that remains in air). Using the protractor, measure the angle of refraction, θ2, of the laser and record in Table 2. 4. Repeat steps 2 and 3 for angles of incidences of 40°,50°, 60°, and 70°. Record the results in Table 2. 5. Calculate sin θ1 and sin θ2 for each trial. Record the results in Table 2. Table 2 Trial 1 2 3 4 5 θ1 (degrees) 30 40 50 60 70 θ2 (degrees) Sin θ1 Sin θ2 6. Draw a graph of the Sin θ1 vs. Sin θ2 on the grid below. Draw in your best-fit line and find the slope. Show slope work below. Ue excel and paste a screenshot of the graph. 7. What does your slope represent? Physics Lab (Online Simulation) 8. Using the chart below of various indices of refraction for various media, identify your mystery material you had in your experiment. 9. Find the percent error of your observed value (slope) using the identified index of refraction as your accepted value. Analysis Questions: 1. Substitute the average value of the index of refraction that you measured in Part A into the equation for index of refraction and calculate the speed of light in the glass. Show work. 2. What if you conducted this experiment (Part A) under water? Compare and contrast the results you get in such a situation to the results you have from this lab. Physics Lab (Online Simulation) Now set up the materials as water on top and air on the bottom. Move the laser back and forth 1. Somewhere between 30o and 60o the behaviour changes. What is the exact angle? 2. What happens at this angle? Physics Lab (Online Simulation) Critical angle When going from a more to a less dense material: The light ray should be ________ away from the _______ At the critical angle the behaviour suddenly changes and the totality of all light is reflected back internally into the water. This is called _________ Complete the table below (use air as the bottom material) TOP Material Water Refractive index (n) critical angle ,θc /o sin ,θc 1 / sin θc 1.33 48 0.743 1.35 Glass Mystery A 1. What effect does increasing the index of refraction have on the critical angle? 2. What equation can be used to calculate the critical angle Physics Lab (Online Simulation) Spectrum On the Prisms page of the simulation phet.colorado.edu/sims/html/bending-light/ latest/bending-light_en.html Select the white on black colour scheme Set up the triangular block as shown. Ensure the ray is pointed near the tip Describe what you see. → Go to the More Tools page of the simulation Use the spectrum to investigate how color changes with wavelength Complete the table Wavelength nm Color 400 450 525 580 600 Yellow 1. Describe the relationship between color and wavelength. 650 700 Physics Lab (Online Simulation) 2. Looking at the spectrum produced by the prism, which colour is bent more? 3. Use this to explain which color slows down the most in a glass block 
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