Lab6_Ray and Wave Optics

Physics-2 Lab 6. Ray and Wave Optics Name: Swarnim Deshwal Name : Arth Sharma Student ID : N01562942 Student ID : N01546699 Objectives  To study the basic optical phenomena, when white light impinges on the interface of two transparent materials: reflection, refraction, dispersion.  Study Snell’s law, determine the optical characteristics of acrylic, e.g., refraction index and critical angle. the rod, unscrew the handle D so that it can be extracted.? Background For light crossing the boundary between two transparent materials, Snell’s Law states n 1 sin θ 1 = n 2 sin θ 2 where θ 1 is the angle of incidence, θ 2 is the angle of refraction, and n 1 and n 2 are the respective indices of refraction of the materials. There is a limiting case of Snell’s law, where light is unable to exit the more dense medium, when the angle of incidence exceeds a so called critical angle (θc) . If the incident angle (θ 1 ) is greater than the critical angle there is no refracted ray and total internal reflection occurs. In this case, Snell’s Law states: n 1 sin θ c = 1sin 90 o Solving for the sine of critical angle gives: sin θ c = 1 n 1 1) we can assume the index of refraction of air ( n 2 in this experiment) is always equal to 1.0. The index of refraction ( n ) depends on the wavelength, or color, of the light. Therefore, the different wavelengths present in an incident ray of white light will be refracted at different angles. The wavelength dependence of a material’s index of refraction is known as dispersion . Humber College Institute of Technology and Advanced Learning Figure 1

Physics-2 List of Equipment and Materials  Light Source  Protractor, white paper, pencil, ruler  Acrylic Trapezoid  Ray Table and D-shaped lens Procedure Part 1: Determine the refraction index of acrylic trapezoid. 1. Place the light source in ray-box mode on a sheet of white paper. Turn the wheel to select a single ray . 2. Place the trapezoid on the paper and position it so the ray passes through the parallel sides as shown in Figure 2. 3. Mark the position of the parallel surfaces of the trapezoid and trace the incident and transmitted rays. Indicate the incoming and the outgoing rays with arrows in the appropriate directions. Carefully mark where the rays enter the trapezoid as point A and leave as point B. 4. Remove the trapezoid and draw a line on the paper connecting points A and B, where the rays entered and left the trapezoid. This line represents the ray inside the trapezoid. 5. Choose either the point where the ray enters the trapezoid (A) or the point where the ray leaves the trapezoid (B). At this point, draw the normal to the surface. 6. Measure the angle of incidence (θi ) and the angle of refraction (θo) with a protractor. Both angles should be measured from the normal. Record the angles in the second row of Table 1. 7. Repeat steps 2–6 with a different angle of incidence. Record your measurement in the third row of Table 1. 8. Repeat steps 2-6 again with a third angle of incidence. Record your measurement in the fourth row of Table 1. The first two columns of Table 1 should now be filled. Note : Suggested incident angle (θi): about 30 o , 45 o , 60 o . Angle of Incidence (θi) Angle of Refraction (θo) Calculated Index of Refraction of Acrylic 30 20 1.466 45 26 1.613 60 40 1.349 Average: 1.476 Table 1 Humber College Institute of Technology and Advanced Learning Figure 2

Physics-2 Part 2. Determine critical angle for total internal reflection 9. Again, place the light source in ray-box mode on a sheet of white paper. Turn the wheel to select a single ray . 10. Position the trapezoid as shown in Figure 3, with the ray entering the trapezoid at least 2 cm from the tip. 11. Rotate the trapezoid until the emerging ray just barely disappears. Just as it disappears, the ray separates into colors. The trapezoid is correctly positioned if the red has just disappeared . 12. Mark the surfaces of the trapezoid. Mark exactly the point on the surface where the ray is internally reflected [Point B]. Also mark the entrance point of the incident ray [Point A], and the exit point of the reflected ray [Point C]. 13. Remove the trapezoid and draw the rays that are incident upon and reflected from the inside surface of the trapezoid. Measure the angle between these rays using a protractor, record in Table 2. 14. Calculate the percent difference between the measured value in step 13 and theoretical values calculated according to Equation 1) (assuming a refractive index of 1.5 for the trapezoid), record in Table 2. Tip : Extend these rays to make the protractor easier to use. Note : this angle is twice the critical angle because the angle of incidence equals the angle of reflection. critical angle (θc): 42 % difference between the measured value and theoretical values: 0.49% Table 2 Humber College Institute of Technology and Advanced Learning Figure 3

Physics-2 Discussion Question 2. How does the brightness of the internally reflected ray change when the incident angle changes from less than θc to greater than θc? Is the critical angle greater for red light or violet light ? What does this tell you about the index of refraction? As the incidence angle increases, the internally reflected ray becomes brighter, and it keeps getting brighter until the incidence angle is equal to the critical angle, where the ray hits it’s maximum intensity. As the incidence angle becomes larger than the critical angle the ray does not become brighter and stays constant. The critical angle of red light is greater than violet light as red light has a higher wavelength which is directly proportional to critical angle. This shows that index of refraction of the same material cause the shorter wavelength to be refracted much farther. Part 3. Dispersion (Optional) 15. Ensure the light source in ray-box mode is on a flat tabletop. Turn the wheel to select a single ray. 16. Put the ray table in front of the light source so the ray from the light source crosses the exact center of the ray table (see Figure 4). 17. Put the acrylic D-shaped lens on the ray table in the marked outline. Turn the ray table so the ray enters the lens through the curved surface, and the angle of incidence is 0°. 18. Hold a piece of white paper vertically near the edge of the Ray Table so the outgoing ray is visible on the paper. 19. Slowly rotate the ray table to increase the angle of incidence. Note : the ray is refracted only at the flat surface of the lens, not at the curved surface. 20. As you continue to increase the angle of incidence, watch the refracted light on the paper. Discussion Question 3. What colors are present in the refracted ray? (Write them in the order of minimum to maximum angle of refraction.) Violet, blue, yellow, orange , white Humber College Institute of Technology and Advanced Learning Figure 4