Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Make nine photocopies of the protractor sheet—three for each activity. Extras may be needed for Activity B if any water is spilled. These sheets may be laminated if desired.
2. Use a ruler to determine the midpoint of the flat side of each semicircular dish. Draw a thin vertical line from top to bottom at this midpoint with a permanent marker.

Safety Precautions

Handle the pins with care. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Lab Hints

• For best results, set up three workstations for each activity throughout the lab. This will allow 9 groups of students to rotate through three activity stations in a 45- to 50-minute lab period. A double lab period (two 45- to 50-minute class periods) will allow time for both an introduction to properties of light before the lab and for a collaborative class discussion after lab.
• Each activity is a self-contained unit and may be completed in any order. Students should need only 10–12 minutes per station—keep the pace fairly brisk to avoid dawdling. Post-Lab Questions may be answered during downtime between stations.
• Prelab Preparation is an essential component of lab safety, and it is also critical for success in the lab. (Standing in front of the lab station is not a good time for students to be reading the activity for the first time.) Having students complete the written prelab assignment and reviewing the safety precautions for each activity will help teachers ensure that students are prepared and can work safely in the lab.
• Have paper towels handy at the Activity B station.
• When light enters the semicircular dish at the midpoint of the flat side (the center of the complete circle), the refracted ray will strike the curved end of the dish perpendicular or “normal” to the surface. Therefore, there will be no refraction of light when the light exits the water at the curved surface. The angle of the light ray traveling in the water will be the same as the light that exits the dish through the curved side. This is why it is important to look through the dish at the center of the flat side of the semicircular dish.
  {12302_Hints_Figure_9}

Teacher Tips

• This activity will fit in well with a physical science unit on light and optics.
• The Water Marbles Demonstration Kit, available from Flinn Scientific (Catalog No. AP7429), is a fun activity to further explore the index of refraction of air and water.

Answers to Prelab Questions

Activity A. Reflections from a Plane Mirror

1. Describe a virtual image as seen in a plane mirror. What is one way a virtual image is different than a real image?

   A virtual image seen in a plane mirror is the same size and vertical orientation as the object. However, the virtual image will be reversed. The image appears to be behind the mirror and can only be seen if one looks in the mirror. A real image can be projected onto a screen.

2. Form a hypothesis to describe the relationship between the angle of incidence and the angle of reflection by completing the following sentence.

   “The angle of incidence is equal to the angle of reflection.”

Activity B. Refraction of Transmitted Light

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

      The speed of light in a vacuum is 3 x 10⁸ m/s.

   2. in air?

      Since the index of refraction of air is 1.00, the speed of light in air is also 3 x 10⁸ m/s.

2. The index of refraction of acrylic glass is 1.5. Use Equation 1 from Activity B to calculate the speed of light as it travels through acrylic glass.

   n = c/v
   v = c/n
   v = 3 x 10⁸ m/s/1.5
   v = 2 x 10⁸ m/s

3. Regarding refraction of light, what is the normal line? Label the normal line in the diagram.

   The normal line is a line perpendicular to the boundary layer between two media.

Activity C. Multiple Images

1. How are the two mirrors positioned for the first trial?

   The two mirrors are placed so the edges of the mirrors touch at the center of the protractor and the mirrors form a 90-degree angle. The reflective sides of the mirrors face in.

   {12302_PreLabAnswers_Figure_10}
2. Where should the cork be placed for each trial?

   The cork should be placed midway between the two mirrors.

Sample Data

Activity A. Reflections from a Plane Mirror

Data Table A

Activity B. Refraction of Transmitted Light

Data Table B Activity C. Multiple Images

Data Table C

Answers to Questions

Activity A. Reflections from a Plane Mirror

1. Based on your observations, compare the size of a virtual image seen in a plane mirror, and its distance from the mirror, compared to the size and distance of the actual object.

   The size of the image is the same as the object. The image appears to be the same distance behind the mirror as the object is in front of the mirror.

2. What does the angle of the line of sight represent when looking at an image in a mirror?

   The angle of the line of sight represents the angle of reflection.

3. Review the hypothesis from Prelab Question 2. The relationship between the angle of incidence and the angle of reflection is known as the Law of Reflection. Based on your observations, write the Law of Reflection.

   When light reflects from a surface, the angle of incidence equals the angle of reflection.

4. Five students are seated at their desks, which are spaced equally apart, in the front row of a classroom (see diagram). The instructor places a large plane mirror on the board directly in front of the middle student. When student 1 looks at the center of the mirror, which student’s image will be seen? Explain.
   {12302_Answers_Figure_11}
   Since the angle of incidence equals the angle of reflection, Student 1 will see the image of Student 5 in the mirror.

Activity B. Refraction of Transmitted Light

5. The index of refraction of water at 20° C is 1.33. Is the speed of light in water less than, greater than or the same as the speed of light in air? Explain.

   The speed of light in water is less than the speed of light in air. The index of refraction of water is greater than that of air because water is more dense than air. According to Equation 1, as the index of refraction increases, the speed of light decreases.

6. In the following diagrams, the letter O at 30° (second trial for each setup) represents the placement of the object, the actual cork and pin. The line and arrow represent the path of incoming light through air or water, respectively, to the center line of the dish. Use the data from Table B to label each diagram as described.
   1. Write the letter T (for toothpick) at the angle that represents each placement of the toothpick when the object was at 30°.
   2. Draw a line with an arrow to show the path of light as it traveled from the center line of the dish to your eye.
   3. Label the angle of refraction.

      Part 1. Air to Water

      {12302_Answers_Figure_12}
      Part 2. Water to Air
      {12302_Answers_Figure_13}
7. As the light travelled from air into water, which way did it bend with respect to the normal line? Did the light speed increase or decrease?

   As the light travelled from air into water, it bent toward the normal as its speed decreased.

8. As the light travelled from water into air, which way did it bend with respect to the normal line? Did the light speed increase or decrease?

   As the light travelled from water into air, it bent away from the normal as its speed increased.

9. The archer fish captures its prey by knocking insects off a branch with a stream of water from its mouth. To compensate for the refraction of light as it is transmitted from air into water, would the fish aim above or below the image of the insect it sees? Explain or draw a diagram.
   {12302_Answers_Figure_14}
   The fish would aim below the image of the insect. As the light reflected from the insect is transmitted from the air into the water to the fish’s eyes, it slows down and bends toward the normal. The image of the insect then appears above the actual location of the insect.

Activity C. Multiple Images

10. What happens to the number of images as the angle between the mirrors decreases? The number of images increases as the angle between the mirrors decreases.
11. A circle is divided into 360 degrees. a. How many 90° sections are in a circle? b. 72° sections? c. 60° sections? d. 40° sections? e. 120° sections?

    a. 4 b. 5 c. 6 d. 9 e. 3

12. Compare the number of images from the data table with the answers to Question 11. What is the relationship between the angle formed by the mirrors and the number of images seen in the mirror?

    The number of images seen in the mirrors always appears to be one less than the number of sections of a circle that has been subdivided into equal parts corresponding to the angle between the two mirrors. For example, four 90° sections are in a circle, and three images were observed when the mirrors were placed 90° apart.

13. Predict how many images would be seen if the angle formed by the mirrors was 20°. If the mirrors were set at a 20° angle, then 17 images would be seen.
14. Write the relationship between the angle formed by the mirrors and the number of images seen in the mirrors as a mathematical equation. Use n to represent the number of images.
    {12302_Answers_Equation_2}

Teacher Handouts

12302_Teacher1.pdf

Introduction

Visible light is a form of energy that is given off by natural or man-made objects such as the Sun or a lightbulb. Other objects may reflect light, enabling them to be seen. Light waves change direction as they are reflected by other objects and may also change speed and direction as they pass from one transparent medium into another. Explore the principles of reflection and refraction as these properties of visible light are investigated.

Concepts

• Reflection
• Refraction
• Transmission
• Multiple images

Background

When light hits a boundary between two different substances, three phenomena can occur—reflection, absorption and transmission. Reflection occurs when a light ray bounces off a surface, such as when light strikes a mirror. Absorption occurs when light energy 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 sunny summer day. The black surface absorbs most of the light energy, which is then converted into heat energy. Transmission occurs when light strikes a new substance and then continues to travel through that substance, such as light that travels in air and then passes into water. As light is transmitted, refraction often occurs. Refraction is the bending of a wave that occurs when it enters a new substance. Most of the time when light strikes a surface, all three phenomena will occur simultaneously. This is why when you look into a calm pool you often will see a faint reflection of yourself as well as what is on the bottom of the pool. The water also gets warmer on a sunny day. In this case, light reflection, transmission, and absorption are all occurring.

Activity A. Reflections from a Plane Mirror
Mirrors are used nearly every day by people when combing their hair or driving a car. Light is partially absorbed and partially reflected off every surface. The manner and amount of light that is absorbed or reflected depends on the type of material and the smoothness of the surface. Mirrors reflect almost all of the incoming light, unlike a black cloth towel, which absorbs much of the incoming light.

A line perpendicular to the reflecting surface is called the normal line. The angle formed between the normal line and the incoming light is called the angle of incidence. The angle formed between the normal line and the reflected light is called the angle of reflection (see Figure 1).

When looking at a reflection from a flat or plane mirror, the reflected image appears to be behind the mirror. The image cannot be seen unless one looks into the mirror. This is known as a virtual image. By contrast, a real image can be projected onto a screen. For a plane mirror, the virtual image is the same size and vertical orientation as the object. However, the virtual image will be reversed. This is why you often see the word AMBULANCE written on the front of these emergency vehicles. When a driver looks into the rearview mirror, the word can be read correctly and the driver can respond accordingly.

Activity B. Refraction of Transmitted Light
When light travels from one transparent medium into another (such as from water into glass or air) at an angle other than perpendicular to the interface between the two different media, the light rays will bend, changing direction. This is known as refraction. 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. Light rays only travel at the “speed of light,” c, in a vacuum. The accepted speed of light in a vacuum is about 3 x 10⁸ m/s. In all other transparent media the speed of light is slower than c. The ratio of the speed of light in a vacuum to the speed of light, v, in another medium is known as the index of refraction of the medium, n (see Equation 1). The index of refraction of a vacuum is exactly equal to 1. The index of refraction of air is very close to that of a vacuum with a value of 1.00293. In most cases, the index of refraction of air is simplified to the value of 1.00.

A line perpendicular to the boundary layer between two media is called the normal line. When light rays hit a boundary layer, the angle formed between the incoming light and the normal line is called the angle of incidence. The angle formed between the light ray after it has entered a new transparent medium and the normal line is the angle of refraction (see Figure 2). Activity C. Multiple Images
When you look in a mirror, you see an image of yourself. As the light is reflected from the mirror to your eyes, your brain perceives the light as if it has traveled in a straight line from beyond the mirror. You perceive your image as if it were behind the mirror. This is known as a virtual image, resulting from an apparent path of light, not the actual path of the light rays. A virtual image cannot be projected onto a screen, whereas a real image can.

Have you ever been in a room with two mirrors? Some interesting observations can be made by exploring virtual images resulting from reflections of two plane (flat) mirrors as they are placed together at different angles.

Experiment Overview

The purpose of this activity-stations lab is to investigate the principles of reflection and refraction of visible light. Three lab activities are set up around the classroom. Each activity focuses on a particular concept associated with the behavior of light as it strikes various boundaries, whether of reflective surfaces or between different transparent media. The activities may be completed in any order.

1. Reflections from a Plane Mirror
2. Refraction of Transmitted Light
3. Multiple Images

Materials

Prelab Questions

Activity A. Reflections from a Plane Mirror
Read through all of Activity A including the Procedure.

1. Describe a virtual image as seen in a plane mirror. What is one way a virtual image is different than a real image?
2. Form a hypothesis to describe the relationship between the angle of incidence and the angle of reflection by completing the following sentence.

   “The angle of incidence is (less than, equal to, greater than) the angle of reflection.”

Activity B. Refraction of Transmitted Light

1. What is the speed of light
   1. in a vacuum?
   2. in air?
2. The index of refraction of acrylic glass is 1.5. Use Equation 1 from Activity B to calculate the speed of light as it travels through acrylic glass.
3. Regarding refraction of light, what is the normal line? Label the normal line in the diagram.
   {12302_PreLab_Figure_3}

Activity C. Multiple Images
Read through the Procedure for Activity C.

1. How are the two mirrors positioned for the first trial?
2. Where should the cork be placed for each trial?

Safety Precautions

One end of the pin is sharp; handle with care. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Activity A. Reflections from a Plane Mirror

1. Insert the pin into the narrow end of one cork and set aside for step 7.
2. Place the mirror in the mirror support (see Figure 4). Note: Make sure the bottom of the mirror support is flat against the work surface so the mirror stands upright without leaning forward or back.
   {12302_Procedure_Figure_4}
3. Place the mirror in the center of the protractor sheet (see Figure 5). The mirror should be on the horizontal line (90°) on the protractor sheet.
   {12302_Procedure_Figure_5}
4. Place the cork without the pin at the 40° mark in Quadrant 4 on the protractor sheet (see Figure 5).
5. Close one eye and look at the reflection of the cork in the mirror from Quadrant 3.
6. Adjust your line of sight so the image of the cork is in line with the center of the protractor. Note the angle on the protractor corresponding to your eye position (line of sight). Record the angle and Quadrant number in Data Table A on the Reflection and Refraction Worksheet.
7. Place the cork with the pin in it behind the mirror—either in Quadrant 1 or 2—at the location where the image of the first cork appears to be. Use the pin as a guide to more accurately position this cork behind the mirror.
8. Look at the angle on the protractor where the cork and pin were placed behind the mirror. Record the angle and Quadrant number in the data table for the cork and pin position. Observe and record the size of the cork with the pin compared to the virtual image. Also note the distance the cork was placed behind the mirror compared to the distance from the first cork to the mirror.
9. Repeat steps 4–8, choosing a different cork position in Quadrant 4. Record the new angle for this position in the data table.
10. Repeat steps 4–8, placing the cork in a different position in Quadrant 3. In what Quadrant will your line of sight be?

Activity B. Refraction of Transmitted Light

1. Fill the semi-circular dish ¾-full with water (approximately 100 mL).
2. Place the protractor sheet on the work surface so Quadrants 1 and 2 are toward the observer.
3. Carefully place the dish on the protractor sheet so the flat side faces Quadrants 3 and 4, the midpoint line on the dish is centered at the intersection of the quadrants, and the curved section is in Quadrants 1 and 2 (see Figure 6).
   {12302_Procedure_Figure_6_Air to water transmission}
4. Insert the pin into the narrow end of the cork.
5. Place the cork at the 50°-mark in Quadrant 4 on the protractor sheet.
6. Close or cover one eye and look through the water-filled dish from Quadrant 1 so your line of sight is from the 50°-mark to the center vertical line on the dish. Note: For best results, keep your eye level at the same height as the tabletop.
7. Are you able to see an image of the cork and pin in the water? Record your observations for 50° in Data Table B on the Reflection and Refraction Worksheet.
8. Keeping one eye closed, move your head and line of sight along the curved part of the dish until the cork-and-pin image appears in the water. Which way did your line of sight move with respect to the normal line (0°)? Add to your observations for 50° in the data table.
9. Continue to move your head and line of sight along the curved part of the dish until the cork-and-pin image in the water lines up with the vertical line on the flat side of the dish.
10. With the image of the pin and vertical line aligned, take the toothpick and place it vertically along the outside of the curved part of the dish so it is aligned with the vertical line and the image of the pin. This will mark your line of sight.
11. Note the angle on the protractor where the toothpick is placed and record the angle and Quadrant number in Data Table B on the worksheet.
12. Repeat steps 5–11 two more times, placing the cork and pin in Quadrant 4 at 30° and 70°, respectively.
13. Carefully rotate the dish and protractor sheet together so the flat side of the dish is toward the observer.
14. Repeat steps 5–11 three times, placing the cork and pin next to the curved side of the dish in Quadrant 1 at 45°, 30° and 10°, respectively. The observer should look through the flat side of the dish, starting from the corresponding angles in Quadrant 4 (see Figure 7). The toothpick should be placed along the circumference of Quadrant 4 to mark the line of sight. Record observations in the “Water to Air” portion of the Data Table.
    {12302_Procedure_Figure_7_Water to air transmission}

Activity C. Multiple Images

1. Place each mirror in a mirror support. Note: Make sure the bottom of the mirror support is flat against the work surface so each mirror stands upright without leaning forward or back.
2. Place the mirrors along the boundary lines of Quadrant 4 on the protractor sheet (0° and 90°) so the edges of the mirrors touch at the center of the protractor with the mirrors forming a 90-degree angle and the reflective sides of the mirrors are facing in (see Figure 8).
   {12302_Procedure_Figure_8}
3. Place the cork on the 45° mark of Quadrant 4 (midway between the two mirrors).
4. Look in the mirrors at eye level with your line of sight from the cork toward the intersection of the two mirrors.
5. Record the number of whole cork images seen in the mirrors for the 90° angle in Data Table C on the Reflection and Refraction Worksheet.
6. Move the mirror from the 90° line to the 72° mark of Quadrant 4, leaving the other mirror at zero.
7. Move the cork so it is midway between the two mirrors (36°).
8. Repeat steps 4–5 for a 72° angle.
9. Repeat steps 6–8 three times, moving the mirror to the 60°, 40° and 120° angles. Remember to adjust the cork position midway between the mirrors for each trial.

Student Worksheet PDF

12302_Student1.pdf