Group Members:   Nicole Reid, Don Benton, John Newchurch

Big Science Ideas/Concepts:      Refraction of Light in different liquids

Possible student misconceptions:

? Light passes through a clear material without being changed in any way.
? When a rainbow spectrum is produced with white light and a prism, the prism somehow adds the color observed.
? By adding more liquid to a container, the ability to view an object at the bottom will not be changed.

Measurable objectives:

? Students will be able to explain that different liquids refract light differently.
? Students will be able to determine the relative degree of refraction of different liquids.
? Students will be able to explain the relationship between the numerical index of refraction and the degree of bending of light in that liquid.

Material required:  (For each participating lab group)
 
Opaque disposable coffee cup (8-10cm high)
Opaque disposable bowl (4-6cm side & 10-15 cm diam.)
One penny    marker or pen
Masking tape    meterstick or tape measure
250 ml rubbing (isopropyl) alcohol
250 ml vegetable oil   250 ml water
200 ml graduated cylinder
Three different colors of food coloring

Intended audience grade level/age:    11 ñ 12 grade

Description of Lesson:
(Including an elicitation of student prior knowledge, a blank student worksheet, an annotated teacherís version and connecting dialogue)

Studentís prior knowledge will include waves through the basics of EMR.  The students will understand the basics of refraction.

INTRODUCTION: The teacher will initially review the basics of refraction.  This will be initiated by using a large beaker of water (or a small aquarium if available) and a long object like a meterstick to demonstrate the apparent ìbendingî of the object when it is placed in the water at about a 45 degree angle.  The students will be asked why the object bends.  The answers will be summarized and written on the blackboard by the teacher.  Further reinforcement of light refraction will be done with a demonstration of a penny at the bottom of an opaque coffee cup slowly filled with water allowing the penny to come into view to a student standing far enough away so he or she cannot see the coin.  The teacher should ask questions during the review and demonstrations to assure students have the basic understanding of what happens with refraction. The fact that different liquids have different indices of refraction is not assumed, and should not be revealed by the teacher at this point.

LAB EXERCISE:
 (Student worksheet for exercise can be found at the end of this lesson plan)
EXERCISE PREPRATION:
1. The three liquids chosen to be tested in the exercise should be placed in three containers marked A, B and C.  Respectively they should be water, rubbing alcohol and vegetable oil.  Place a small amount of food coloring (a different color for each lab group) in the water.  Do not put in very much coloring, as the water must be very transparent for the students to perform the lab.  Do not tell the students the identity of the liquids at this time.
2. When introducing the lab exercise, the teacher should emphasize the importance of students attempting to explain what they observe.  Also, they should be using scientific methods when doing any analysis.  The goal of the exercise (which is not revealed to the students until after the lab portion) is for the students to see light refract differently in each liquid, and be able to rank them from highest to lowest amount of refraction.  At the end of the exercise, the teacher will establish the relationship between the numerical index of refraction and the actual refraction of light in any particular liquid.
3. Each lab group will determine their own methods for proving their prediction.  As the teacher walks around the classroom during the exercise, he/she can question the students about their procedures and guide them if necessary.

EXAMPLES:
? One group may use the cup and not the bowl.
? Some may vary the amount of fluid used in the container and keep the distance from the observer constant.
? Others may vary the distance to the container and keep the amount of liquid constant in the container.
? Any method is acceptable.  However, the indices of refraction of water and rubbing alcohol are very close.  Unless the students are very careful in their measurements, they will appear the same.  If they use the bowl with it set on a table, the viewing angle will be very narrow requiring the student to stand quite a ways away from the bowl before the coin at the bottom disappears.  This will magnify small differences in the angle of refraction and allow them to clearly see the difference between these two liquids.  They should have no problem seeing the difference between either water or rubbing alcohol and vegetable oil.
4. During the exercise, students should be documenting their observations as well as the method they have chosen.  A data table is suggested for recording their observations.  Again, the teacher can make suggestions where warranted.

The teacher will organize groups, explain what equipment will be given out, and detail what students will be expected to do.  This part will be a full inquiry lab to allow the students to discover that different liquids have different degrees of refraction.  The method they chose to do this will also be a part of this lab.  Allow about 20 minutes for the lab.

Handout will be based on the question:
   Does light refract the same in different liquids?
Criteria for Exercise:
Make a prediction.
 Determine a method to prove or disprove your prediction.
 Construct a data table.
Perform at least two measurements with each liquid given within the 20-minute time limit.
 Draw a conclusion.

CONCLUSION (OR WRAP-UP) The teacher will initially question each group.  A spokesperson will verbally give their predictions and findings.  The teacher will write each answer on the blackboard.  Each question should be written briefly with space left below for each groupís answer.  The questions should include: What was your prediction?  What was your method of proving (or disproving) your prediction?  Were you able to prove your prediction ñ was it correct?  Was your data conclusive or could it have been interpreted in any other way?  If you were not able to prove your prediction, why not?  What could you have done differently in order to conclusively prove it?

Suggested format for listing on blackboard:

Prediction Proved or Not Method Improvements
No change in refraction for different liquids Disproved Used bowls and measured distance to point not seen Always use same height person ñ place coin in center

Different refraction in each liquid Proved Used coffee cups and measured height & distance to point not seen Bowls would be better as small measurements were difficult

Etc.

By this point it should become clear that different liquids do refract light differently. The teacher should now absolutely state that different materials always refract light differently.

Finally, the teacher should now ask which liquid ìAî, ìBî or ìCî had the greatest effect on light.  Which liquid refracted light the most?  Again a table should be written on the board listing each groups results.  At this point, the identity of each liquid can be revealed.  Also, the index of refraction can be written above each liquid (they are approximately: Air ñ 1.00 Water ñ 1.33 Rubbing alcohol ñ 1.38 and Vegetable oil ñ 1.47) The students findings should show that the greater the index of refraction, the greater the deflection (or bending) of light observed.  The index of refraction should be briefly defined as the speed of light in a vacuum divided by the speed of light in a material (n = c/v).

The teacher should note that if a student were to look up the physical properties of any clear material in any chemical handbook, the index of refraction would be listed.   As a note, here in South Louisiana, many books list the different indices of refraction for different solutions of cane sugar.  This is one of the ways our local sugar mills test the sugar they produce from our numerous cane fields each fall.

For final closure, the teacher should restate that as the speed of light slows down or speeds up when it moves from any material to another, the direction of a beam of light would be slightly changed.  This is refraction.

A possible extension:

Prove or demonstrate Snellís Law.  Determine actual angles of refraction and calculate indices of refraction.  Do lab results agree with accepted values?

A relevant internet (or other) resource(and a brief description of how it could enhance the lesson):

LASER JELLO
Address:
http://netra.exploratorium.edu/wsw/progress_snacks/laser_jello/index.html
Application: This site presents a write up for an exercise dealing with refraction plus internal reflection.  The medium used is Jell-O so the reflected and refracted beams are easily seen.  This is a great extension exercise.

PHYSICS EXPERIMENT: THE REFRACTION OF LIGHT
Address: http://www.batesville.k12.in.us/Physics/PhyNet/Optics/Refraction/Labs/Refraction_Lab.html
Application: This site contains another lab exercise or demonstration of refraction.  The two questions this exercise deals with are: 1. When light refracts when passing from one medium to another, how is the angle of incidence related to the angle of refraction?  2. Does light refract at different angles in different substances?

End of lesson assessment:

Attached
 PHYSICS EXERCISE WORKSHEET
 

QUESTION: Does light refract the same in different liquids?

After the activity is complete, each student will turn in a written lab report, which must include the details for the criteria below.

CRITERIA FOR EXERCISE:
Predict a conclusion.
Determine a method to prove or disprove your prediction.
Construct a data table.
Perform at least two measurements with each liquid keeping in mind the 20-minute time limit.
Draw a conclusion.
 REFRACTION OF LIGHT IN LIQUIDS - ASSESSMENT
 

Each student will turn in a lab report as detailed before.
The questions below should be answered and turned in as part of the lab report.

QUESTIONS:
1. Did you conclusively prove or disprove your prediction?  Was your conclusion in line with what should have happened?  How could you change your procedure to improve your results?
 
 
 
 
 
 
 
 

2. Based upon the lab and the class discussion, explain what the index of refraction of a liquid tells you about that liquid?
 
 
 
 
 
 
 

3. If you have two liquids; one with an index of refraction 1.67 and the other with an index of refraction of 1.22 ñ which liquid would cause light to refract the least?
 
 
 
 
 
 

4. Describe where you would appear to a fish looking at you from under water.  Draw a sketch.
 

 IDEAL ASSESSMENT RESPONSES

1. If the student predicted no difference in the refraction of liquids, this exercise should conclusively disprove that notion.  In this instance, they should have been able to redefine their procedure to allow for the desired discovery.  If they correctly
ranked the liquids from most refraction to least refraction, then the lab and discussion should just confirm their prediction.
 

2. The index of refraction defines the degree of light ìbendingî you would observe in any particular liquid. This is a measurable physical property of all transparent materials.
 
 

3. The liquid with the index of refraction of 1.22 should refract the least.
 
 

4. The students should indicate that the image of the person is not actually where the person is located. The main idea that the students should be able to explain is that light refracts when going from one medium to another, and they should be able to draw a diagram to illustrate this.  They probably will not be able to predict that light will refract differently when traveling from a higher optical density material (water) to a lower optical density material (air).