annotated-LAB_7_20PHYSICS_20GABRIELA_20

Physics Lab (Online Simulation) Light Reflection & Refraction Critical Angle, Spectrum Electricity and Light TA name: Shreejan Khanal Due Date: 3/21/22 Student Name: Gabriela Weakley Student ID: 1001964143 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? N=c/v n= index of refraction for a medium c=speed of light un a vacuum v=speed of light in the medium 2) What does the term Normal mean in the context of dealing with an optical surface? Normal to an optical surface typically infers to the perpendicular straight line imagined on the optical interface. Normal=perpendicular 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) When light refracts from a less dense to denser medium, the ray will reflect towards the Normal. From a denser to a less dense medium The ray deflects away from the normal when travelling from a lighter medium to a denser one. 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? For a convex lens, the focal point is situated right to the lens and for concave lens, the focal point is situated left to the lens. 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]

Physics Lab (Online Simulation) where R is the radius of curvature. 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. n 1 sin  1 = n 2 sin  2 , and [4] 2.  1 and  2 are coplanar. principal axis f R radius of curvature Figure 2 Normal Normal medium 1  1  2 medium 2 medium 1 Normal  3 =  1  2 n 1 n 1 n 2

Physics Lab (Online Simulation) Figure 3 represents two interfaces the first is from a lower to higher index of refraction (medium 1 (n 1 ) < medium 2 (n 2 )), and the second is from a higher to low index of refraction (medium 2 (n 2 ) > medium 1 (n 1 )). At the interface of n 1 < n 2 the angle  2 will be less than  1 in relation to the normal which is in agreement with equation [4]. Likewise when n 2 > n 1 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: [5] In equation [5], (n 2 /n 1 ) is called “the index of refraction of medium 2 relative to medium 1”. Note that if medium 2 is air then n 2 = 1 and that n 1 can be found by n 1 = 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.              1 2 1 c 1 2 c 2 c 1 n n sin or , n n sin or , 90 sin n sin n Figure 4 Normal denser medium lesser medium  c  2 = 90  Figure 3

Physics Lab (Online Simulation) 1 1.50 30 26 .50 .438 1.14 2 1.50 10 8 .173 .139 1.24 3 1.50 15 13 .258 .224 1.15 4 1.50 40 34 .642 .559 1.14 5 1.50 60 50 .866 .766 1.13 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? As it passed into the medium with a higher index of refraction, the light reflected towards the normal 2. Calculate sin θ 1 and sin θ 2 for each trial. Record the results in Table 1. 3. Calculate n 2 (glass) for each trial. Record the results in Table 1. 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? Yes, there is a good agreement between the values. All values are around 1.5. 5. Compare your calculated n 2 with the given index of refraction given in the lecture notes, n glass . Do they agree? Explain why it does or doesn’t. Calculate the percentage difference. They do agree. The value for n2 is about 1.5 and the refractive index of nglass is 1.5

Physics Lab (Online Simulation) 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, θ 1 , at 30°. 3. Ignore the reflected ray (the ray that remains in air). Using the protractor, measure the angle of refraction, θ 2 , 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 θ 1 and sin θ 2 for each trial. Record the results in Table 2. Table 2 Trial θ 1 (degrees) θ 2 (degrees) Sin θ 1 Sin θ 2 1 30 10 0.5 .173 2 40 15 0.642 .258 3 50 19 0.77 .309 4 60 20 .866 .342 5 70 22 .94 .370 6. Draw a graph of the Sin θ 1 vs. Sin θ 2 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. Slope= y2-y1/x2-x1 = (.94-.5)/(.370-.173)= 2.22

Physics Lab (Online Simulation) 9. Find the percent error of your observed value (slope) using the identified index of refraction as your accepted value. (2.22-2.16)/2.16x100 = 2.78 percent 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. V=c/n=3x10^8/1.55=1.99x10^8= 2x10^8 m/s 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. There wouldn’t be a reflected ray if this experiment were conducted under water.

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 30 o and 60 o the behaviour changes. What is the exact angle? For air and water the angle is 48.7 degrees 2. What happens at this angle? The incident ray starts to move through the surface of separation of water and air, and above this angle total internal reflection takes place.

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.  The white light goes through the triangular block and bends into different colors Go to the More Tools page of the simulation Use the spectrum to investigate how color changes with wavelength Complete the table Wavelength nm 400 450 525 580 600 650 700

Physics Lab (Online Simulation) Color violet blue green Yellow orange red Dark red 1. Describe the relationship between color and wavelength. As wavelength is increased, colors change in reverse order of ROYGBIV 2. Looking at the spectrum produced by the prism, which colour is bent more?