Materials Included In Kit

Additional Materials Required

Prelab Preparation

Note: This will make one gelatin refraction dish.

1. Pour 100 mL of deionized water into a 150-mL beaker.
2. Place the beaker on a hot plate and heat until the water reaches a low boil.
3. Remove the beaker from the hot plate and add 5 drops of red food coloring
4. Using a glass stirring rod, slowly stir 4 g of gelatin into the colored water. Continue stirring until all the gelatin has dissolved.
5. Pour the liquid gelatin solution into the semicircular refraction dish and cover with plastic wrap.
6. Let stand for 3–4 hours or until the gelatin has solidified. This will occur more quickly if placed in a refrigerator.

Safety Precautions

Remind students to not aim the laser pointer directly into anyone’s eyes. The low-power, coherent light can cause damage to the sensitive retina and may lead to permanent eye damage. Prevent stray laser light from projecting beyond the classroom to eliminate any unintentional exposure to the laser light. When refracting the laser light, it is best to do this on a low work surface to keep the refracted laser light below “normal” eye level. For people with sensitive eyes it is recommended that dark, IR-protective, safety glasses be worn. Follow all other normal laboratory safety guidelines. When preparing the gelatin, always wear chemical splash goggles and a lab apron

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Gelatin can be disposed of according to Flinn Suggested Disposal Method #26a. All other materials can be saved for future use.

Lab Hints

• Enough materials are provided in this kit for eight groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The questions can be answered after class.
• Prepare the gelatin the night before the lab activity to ensure that it has solidified. Store in a refrigerator overnight.
• One laser pointer is provided in this kit. Additional laser pointers may be purchased from Flinn Scientific (Catalog No. AP8934).
• The use of lasers in the classroom has significant educational value, and the safe use of lasers in the classroom is possible. Please remind students how to safely use a laser. 

Teacher Tips

• Before performing this lab activity, students should be familiar with the terms angle of incidence and angle of refraction.
• Take this activity a step farther by having the students test the effects of laser light traveling through different colors of gelatin. By shining red laser light through different colors of gelatin, the students will see that certain colors of gelatin will allow the light to be transmitted (red, orange), while other colors will absorb the laser light (blue).
• For further concept development of this topic, try the Meter Stick Optics Bench Kit (Catalog No. AP6098), Modeling Eye Optics (Catalog No. AP6768), and Optics Kit (Catalog No. AP9043) available from Flinn Scientific.

Answers to Prelab Questions

1. What is the speed of light
   1. In a vacuum?

      The speed of light in a vacuum is 3.0 x 10⁸ m/s.

   2. In air?

      The speed of light in air is also 3.0 x 10⁸ m/s.

2. Using Equation 1 and the answers to Questions 1, calculate the index of refraction (n) for light in air.
   {12727_Background_Equation_1}
   {12727_PreLabAnswers_Equation_4}
3. Using Equation 2, the answer to Question 2, and the angles given in Figure 1, calculate the index of refraction for light in water (n₂).
   {12727_Background_Equation_2}
   {12727_PreLabAnswers_Equation_5}
4. Using Equation 1 and your answer above, calculate the speed of light in water (v).
   {12727_Background_Equation_1}
   {12727_PreLabAnswers_Equation_6}
5. Compare the speed of light in water to the speed of light in air. In which substance is the speed of light slower?

   The speed of light is slower in water than in air. This is because the density of water is greater than air.

Sample Data

Part 1

Part 2

Answers to Questions

Part 1

1. Look at the diagram Air into Gelatin. Is each refracted ray closer to the 0° line or farther away from the 0° line compared to its corresponding incident ray?

   Each refracted ray is closer to the 0° line compared to its corresponding incident ray.

2. Using Formula 2 and the data from the table, calculate the index of refraction “n₂” for each ray. The index of refraction of air is needed in order to complete this calculation. See the answer to Question 2 in the Prelab Questions section. Show all work for the calculation of A. Record the answers for A–G in the data table.

   n₁ sinθ₁ = n₂ sin θ₂
   1 sin 10° = n₂ sin 8°
   n₂ = 1.25

3. Calculate the average index of refraction for light in gelatin. Record this value in the data table.

   1.25 + 1.32 + 1.28 + 1.32 + 1.37 + 1.34 + 1.32 / 7 = 1.31

4. Using the average of the index of refraction for gelatin and Formula 1 from the Background section, calculate the speed of light in gelatin.
   {12727_Answers_Equation_6}
5. Is the speed of light in gelatin greater or less than the speed of light in air?

   The speed of light in gelatin is less than the speed of light in air.

6. Look at the speed of light in air and its index of refraction. Next, look at the speed of light in gelatin and its index of refraction. What is the relationship between the index of refraction and the speed of light in a substance? How does this agree with the formula n = c/v?

   As the index of refraction increases, the speed of light decreases. This agrees with the formula which represents in indirect proportion between the two.

Part 2

7. Look at the diagram Gelatin into Air. Is each refracted ray closer to the 0° line or farther away from the 0° line compared to its corresponding incident ray? Explain the results.

   Most refracted rays are moving away from the 0° line while others are exactly the same as the incident ray. The rays that are the same, however, are reflected, and they show up in a different quadrant on the diagram than the refracted rays.

8. According to the diagram Gelatin into Air, at what angle of incidence was total internal reflection first observed?

   Total internal reflection was first observed when the angle of incidence was 50° on the diagram.

9. Total internal reflection occurs when the angle of incidence is greater than the critical angle. Based on your data, between what two angles of incidence does the critical angle occur for gelatin and air?

   The critical angle falls between 40° and 50°.

10. The critical angle of a substance is the angle of incidence (θ₁) at which the angle of refraction is 90°. This can only occur when the speed of light increases as it travels from one substance into another. Calculate the critical angle (θ₁) between gelatin and air using the formula below. Remember, in this part of the experiment gelatin is substance 1, therefore n1 is the index of refraction for gelatin (average calculated in Part 1) and n₂ is the index of refraction of air (calculated in the Prelab Questions).

    n₁ sin θ₁ = n₂ sin 90°
    1.31 sin θ₁ = 1 sin 90°
    θ₁ = 49°

11. Does the critical angle calculated above agree with the results on the diagram Gelatin into Air? Explain.

    Yes, the critical angle calculated above agrees with the diagram. The critical angle should fall between 40° and 50° and the calculation shows that it does.

12. Look at both diagrams—Air into Gelatin and Gelatin into Air. In what diagram is the speed of light increasing as it enters substance 2? In what diagram is the speed of light decreasing as it enters substance 2? Explain.

    The speed of light increases as it travels from gelatin into air because air is less dense than gelatin. The speed of light decreases as it travels from air into gelatin because gelatin is more dense than air.

13. What relationship can be made between the direction of the refracted ray of light (closer to, or away from the 0° line), and the increase or decrease in the speed of light?

    When light bends closer to the 0° line, this means light has slowed down. When light bends away from the 0° line, the speed of light has increased.

References

www.exploratorium.edu (accessed March 2008)

Introduction

Why does a straw sitting in a glass of water look broken at the water’s surface? Why does a diamond sparkle? These unique effects are due to a change in the speed of light as it passes from one medium into another. This phenomenon is known as refraction.

Concepts

• Index of refraction
• Reflection
• Snell’s law
• Absorption
• Refraction
• Transmission

Background

When light hits a boundary between two different substances, three phenomena can occur—reflection, absorption and/or transmission. Reflection is when a wave bounces off a surface, such as when light strikes a mirror. Absorption is when light is retained in a substance, and changed into another type of energy such as heat. A classic example of this is a black surface becoming extremely hot on a summer day. The black surface absorbs light energy that is then converted into heat energy. Transmission is when light strikes a new substance, and then continues travel through that substance, such as light that travels in air then passes into water. Most of the time when light strikes a surface, all three phenomena will occur simultaneously. This is why, when looking out a window, you will often see a faint reflection of yourself while at the same time you can see through the window. In this case, reflection and transmission are both occurring.

This lab activity focuses on the transmission of light. As light is transmitted, refraction often occurs. Refraction is the bending of a wave that occurs when it enters a new substance. This bending occurs because the speed of light changes when it passes from one substance to another, due to a difference in the densities of the two substances. The accepted speed of light in a vacuum and in air is about 3 x 10⁸ m/s. When light travels from air into a substance with a higher density, the speed of light will decrease, and vice versa. The speed of light in a substance other than air can be calculated using Equation 1. In order to use this formula, the index of refraction (n) for the substance must be known.

n = index of refraction
c = speed of light in a vacuum
v = speed of light in a substance

In 1621, Dutch astronomer and mathematician Willebrord Snellius (1580–1626) came up with a quantitative law of refraction, which is known today as Snell’s Law. Equation 2 represents Snell’s Law where n₁ and n₂ are the indices of refraction for the substances on either side of a boundary, θ₁ is the angle of incidence, and θ₂ is the angle of refraction. See Figure 1 which represents a ray of light traveling from air into water. If three of the values in this formula are known, the fourth can then be calculated. This formula can therefore be used to find the index of refraction for an unknown substance, but only if the index of refraction (n) for the other substance is known. Once the index of refraction (n) for the unknown is found, the speed of light in this substance can be calculated using Equation 1.

n₁ = index of refraction for substance 1
n₂ = index of refraction for substance 2
θ₁ = angle of incidence
θ₂ = angle of refraction

When the speed of light increases as it travels from one substance into another, an unusual phenomenon may occur. At a specific angle of incidence, light will refract and skim the surface between the two substances. This means that the refracted ray of light will be at a 90° angle from the 0° line (see Figure 2). This specific angle of incidence is known as the critical angle (see Figure 2, which represents a ray of light traveling from water into air). The critical angle (a specific angle of incidence) for light at a water–air boundary is about 48.8°. If the incident ray of light strikes the boundary at an angle greater than this critical angle (48.8°), the ray will not enter the second substance, and it will experience total internal reflection (see Figure 3). Total internal reflection means that 100% of the light is reflected back into the first substance and none of the light travels into the second substance. The light behaves as if it is reflecting off the surface of a mirror, obeying the law of reflection. Total internal reflection is what causes a diamond to sparkle. Most of the light inside a diamond cannot escape because a diamond has a critical angle of about 24.4° (see Figure 4). Therefore if light inside a diamond hits the diamond–air boundary at an angle greater than 24.4°, the light inside the diamond will not escape and it will be reflected back inside the diamond. The light will continue to bounce around inside the diamond, creating a sparkling effect, until it hits the boundary at an angle less than the critical angle, at which point the light will pass from the diamond into the air.

Experiment Overview

The purpose of this activity is to observe the total internal reflection of light and measure the refraction of light as its speed increases and decreases.

Materials

Prelab Questions

1. What is the speed of light
   1. In a vacuum?
   2. In air?
2. Using Equation 1 and the answers to Question 1, calculate the index of refraction (n) for light in air.
3. Using Equation 2, the answer to Question 2, and the angles given in Figure 1, calculate the index of refraction for light in water (n₂).
4. Using Equation 1 and your answer above, calculate the speed of light in water (v).
5. Compare the speed of light in water to the speed of light in air. In which substance is the speed of light slower? Explain.

Safety Precautions

Do not aim the laser pointer directly into anyone’s eyes. The low-power, coherent light can cause damage to the sensitive retina and may lead to permanent eye damage. Prevent stray laser light from projecting beyond the classroom to eliminate any unintentional exposure to the laser light. When refracting the laser light, it is best to do this on a low work surface to keep the refracted laser light below “normal” eye level. For people with sensitive eyes it is recommended that dark, IR-protective, safety glasses be worn. Follow all other normal laboratory safety guidelines.

Procedure

Part 1

1. Place a semicircular dish containing gelatin on the proper section of the diagram titled Air into Gelatin (see Figure 5). Make sure the center of the dish is at the intersection of 0° and 90°.
   {12727_Procedure_Figure_5}
2. Open a manila folder and stand it vertically at the round side of the semicircular dish as shown in Figure 5. The manila folder should act as a backstop for the laser light.
3. Place the laser pointer on the line labeled A 10°. Squeeze the button on the pointer and aim the laser beam at the intersection of 0° and 90°.
4. Observe the laser beam travel from the air into the gelatin. Using a pencil, place a mark on the sheet where the laser light exits the gelatin on the curved side of the dish. Write the letter A1 next to this mark.
5. Repeat steps 3 and 4 for the incident angles B–G marked on the diagram.
6. Remove the semicircular tray, and draw a line from the intersection of 0° and 90°, to the mark labeled A₁, where the laser light exits the gelatin.
7. Repeat step 6 for the pencil marks labeled B₁–G₁.
8. Record each refracted angle (A₁–G₁) in the proper section of the data table.

Part 2

9. Place a semicircular dish containing gelatin on the corresponding section of the diagram titled Gelatin into Air (see Figure 6). Make sure the center of the dish is at the intersection of 0° and 90°.
   {12727_Procedure_Figure_6}
10. Repeat steps 2–7 from Part 1, using the diagram titled Gelatin into Air. For this diagram, light travels from gelatin into air. Remember to set up the manila folder. This folder should act as a backstop for the laser light. It will also help to see where the light exits the gelatin. Adjust the manila folder as you proceed from points A to G in order to make sure that laser light does not travel beyond your work area toward others.
11. See your teacher for proper disposal procedures.

Student Worksheet PDF

12727_Student1.pdf