Lab 4 Reflection-Refraction and Total Internal Reflection

Physics LAB 4: Ray Tracing, Laws of refraction and Snell’s Law using a Virtual Setup: Virtual Lab Name: Vianey Quintanilla, Lilibeth Mercado, Lina Garza, Karina Garza Introduction: Background on Normal, Incident angle, reflected and Refracted ray : When light strikes an interface, some of the light bounces off the surface, and some will enter the glass. The light that bounces off the surface is said to have been “reflected”, or to have undergone reflection. The light that enters the glass is said to have been “refracted”, or to have undergone refraction. When we examine the properties of transparent medium like glass, it is important to first realize that glass work on the principle of reflection/refraction and that lenses (and prisms) work on the principle of refraction. Where mirror works on the principle of reflection only. Lenses and mirrors are often used to magnify or reduce images. To do this the light will be either focused to a real focal point (like in a concave mirror or a converging lens) or spread away from a virtual focal point (like in a convex mirror or a diverging lens). At first, we will be working on to verify the laws of reflection. Figure: Reflection and refraction of light at the interface of air and glass Activity 1 : Go to this is the link : https://phet.colorado.edu/en/simulation/bending-light When the site opens, scroll down until you see a light blue circle with PHYICS inside. Click this circle. Then select Light & Radiation. Next, find the App called “Bending of Light.” Finally, click on the app (you may click on the Play button or the Red 5) You will start the lab using the Intro section. Preliminary Investigation 1. Turn the laser on by clicking the red button. You will see light getting reflected and refracted, but you will be focusing only on reflected ray for this part of the experiment. Now move the laser [turn over]

pointer left and right to change the angle of incidence as shown in figure and verify the laws of reflection. Incident Angle (Degrees)= Reflected Angles (Degrees) 15 15 30 30 45 45 60 60 2. What are some observations you make on angle of reflection when you change the angle of incidence? Describe them, ( answer, explain, evidence). - The angle of reflection is the same as the angle of incidence. For example, the angle of incidence at 15 degrees gave us a reflected angle of 15 degrees. - The dark red line represents the angle of incidence at 15 degrees, and the light red line represents the angle of reflection at 15 degrees. 3. What are the laws of reflections? Do you think, laws of reflection hold in your experiment? if it holds describe, ( answer, explain, evidence). - The INCIDENT ray, REFLECTED ray, REFRACTED ray and the NORMAL at the point of incidence all lie in the same plane. Activity 2 : Go to this is the link : https://phet.colorado.edu/en/simulation/bending-light When the site opens, scroll down until you see a light blue circle with PHYICS inside. Click this circle. Then select Light & Radiation. Next, find the App called “Bending of Light.” Finally, click on the app (you may click on the Play button or the Red 5) You will start the lab using the Intro section. Preliminary Investigation 4. Turn the laser on by clicking the red button. You will see light getting reflected and refracted. Now move the laser pointer left and right by clicking and holding on the silver part and moving it. What are some observations you make? Describe them, ( answer, explain, evidence). As you move the laser left and right you see the refraction line. But if you keep the laser in the vertical or horizontal line you see no refraction. JDC – Created 05/02/2019 – Printed 14/06/2020

6. Put the initial material back to Air. Now click on water in the lower material. Change it to several other materials and made record your observations. Is there are way to make the light, go straight from AIR into the lower material, ( answer, explain, evidence). Yes, there is a way for light to go straight if you have the light pointing straight down at a 0 degree angle or at a 90 degree angle. JDC – Created 05/02/2019 – Printed 14/06/2020

PART I 1. For this part of the lab you will stay in the intro section. 2. Keep the starting material AIR, and the lower material WATER. 3. Drag the yellow protractor onto the experimental area. Move it so the AIR/WATER barrier line is just on 90°. The dotted Normal line must be at 0°. 4. Now you will change the incident angle to the angles listed on the table below. Use the protractor to set the Incident angle and find the Refracted angle. Incident Angle (Degrees)= Refracted Angles (Degrees) 0 0 15 11 30 22 45 32 60 41 75 46 90 90 5. Using Snell’s law pick three incident angle and check to see if you PHET gave you the correct refracted angles. Show must your work . n 1sin θ 1 = n 2 sin θ 2 ,You willbe solvingfor θ 2 JDC – Created 05/02/2019 – Printed 14/06/2020

Step 2 Gradually increase the angle of incidence until i = 40°. The cropped screenshot on the right should help. Read off and record the angles of reflection and refraction below . Angle of reflection = …… …40… ……… ° Angle of refraction = … …… 55… ……… ° Step 3 Keep increasing the angle of incidence until the angle of refraction is as close to 90° as you can get it. (See the cropped screenshot on the right) If you increase the angle of incidence further then the refracted ray will disappear. The angle when this happens is called the “ critical angle ” for water. Record the critical angle for water here : Critical angle for water = 49 JDC – Created 05/02/2019 – Printed 14/06/2020

Step 4 Return the angle of incidence to 0° and change the material in the top half to glass. Repeat step 3 to find the critical angle for glass. Record it here : Critical angle for glass = 40 ° Step 5 Repeat the process to find the critical angle for material Mystery A and Mystery B. Record the results here : Critical angle for Mystery A = ……… 49 ………… ° Critical angle for Mystery B = …… 68 …………… ° Step 6 Put the critical angle results into the table below and complete the other columns using the instructions below the table. Material critical angle, C / degrees sin ( C ) 1/sin( C ) refractive index, n Water 49 0.755 1.33 1.33 Glass 40 0.75 1.55 1.55 Mystery A 49 0.755 1.33 ? Mystery B 68 0.93 1.08 ? JDC – Created 05/02/2019 – Printed 14/06/2020 1. Get this number from your work earlier on this page 3. Now press the x –1 button. 4. Get this number from the PhET sim (see example below) 2. On a calculator press the sin button, then type the critical angle, then press the = button. Step 7 Look at the last two columns of the table and use what you see to suggest n for Mystery A and B.

Activity 4: Intensity of Reflected ray Step 1 Set the PhET sim up so that it looks like the screenshot below. The material in the top half should be water , the material in the bottom half should be air , and the angle of incidence should be 10°. Catch the reflected ray with the intensity meter as shown in the screenshot. Step 2 Increase the angle of incidence by 10°, catch the reflected beam with the intensity meter to find out what % intensity the reflected beam has. Record your result in the table on the right. Step 3 Complete the rest of the table by making measurements from the sim. Step 4 When the angle of incidence is greater than the critical angle, 100% of the light intensity is reflected. This is called total internal reflection because all the light is reflected. Use the data in the table to suggest a value for the critical angle in water, and explain why you have chosen that angle The Critical angle for water is 49 degrees because at an angle of 50 degrees for the angle of incidence, 100% of the light intensity is reflected. At 49 degrees the angle of incidence is still less than the critical angle meaning that not all light will be reflected. JDC – Created 05/02/2019 – Printed 14/06/2020 angle of incidence / degrees intensity of reflected ray / % 10 2.19 20 2.84 30 4.60 35 6.08 40 9.73 45 18.69 50 100 55 100 60 100 70 100 80 100

…………………………………………………….…………………………………………………….………………………………………………………………………. …………………………………………………….…………………………………………………….………………………………………………………………………. Activity 5: Dispersion in Prism The PhET sim also has two other options: Prisms and More Tools . Play with these and record the best things you find out in the boxes below. The first image shows light being reflected on air. The light being reflected on air, glass, and water all differ even though the same prism(circular) is being used. Second image shows light being reflected and refracted in water. Third image shows light being reflected and refracted on glass. There is a small difference between the reflected and refracted lines from the second image to the third. JDC – Created 05/02/2019 – Printed 14/06/2020