University of Virginia Physics Department Polarized Filters A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.9

Objectives

Students will

• Observe the properties of light.
• Observe the relationship between the amount of light that passes through polarized filters and the orientation of those filters.

Motivation for Learning

Teacher Demonstration

Materials

• a clear tank that can hold several gallons of water (5 gallon aquarium is ideal)
• a silvery, gray object (such as a lead fishing weight)
• at least 2 pair of sunglasses, one polarized and one not
• a light source to shine down on the water

Procedure

1. Lead the class into the discussion by asking why they wear sunglasses. Ask students how sunglasses "work." Many will probably say that they block the light. Discuss what "light" is. Is it a particle or a wave?
2. The tank of water should be set up in the front of the room. Shine a light source down onto the water to simulate the sun shining on the water. Ask the first student volunteer to look from the top of the tank down into the water first with the non-polarized lenses. Ask how easy it is to see the object on the bottom of the tank. The student should respond that it is rather difficult to see the object.
3. Direct the same student to look into the tank now using the polarized glasses. Ask them to report how much easier it is to see the object on the bottom of the tank. Allow all the students to come up and observe the effects of the different glasses.
4. Discuss why it might be advantageous for people, especially fishermen, lifeguards, or photographers to use polarized glasses or lenses.

Background Information

Students need a basic understanding of the properties of light. Shadows are created because light is reflected or "bounced" off an object. In this way, light acts much like a particle. It is sometimes compared to "little bullets" or "rays" that stream out from a source. Visible light, what our eyes detect, is only a small part of the electromagnetic spectrum. The electromagnetic spectrum ranges from very long waves such as radio waves to very short waves such as gamma rays. Opaque materials absorb or reflect all visible light, and you cannot see objects through them. Transparent materials allow light to pass through them and you can see the object through them. Translucent materials allow some light to pass through.

Reflection occurs with all types of electromagnetic waves, and when light strikes a mirror we are able to see our reflection. Sometimes light waves are bent as they speed up or slow down as their speed changes when they move from one medium to another. Example: Light slows down as it enters water from the air. That's why an object may appear "bent" or "broken" at the water's surface. These are some ways in which light behaves like a wave.

The light that comes from most sources is a wave that vibrates in many directions perpendicular to the direction of propagation. If light is passed through a polarized filter these transverse light waves will only vibrate in one direction. The filter contains molecules that act like parallel slits that allow those light waves vibrating in one direction to pass through. This principle is sometimes compared to venetian blinds where light is blocked from certain angles and allowed to pass through others. If a second filter is placed so that its molecules are at a right angle to the first filter, all the light should be blocked. Light that is reflected off horizontal surfaces (bodies of water, metal surfaces) is partially polarized horizontally. Because of this, polarized sunglasses are made with vertically polarizing filters to block out glare while allowing all of the vertically polarized light to pass through. Photographers also rely on polarized filters for camera lenses to reduce the glare in photographs.

In this activity, students will use polarized filters in different positions to detect the amount of light that passes through them using a light probe with the computer (or a graphing calculator). Teachers need to be sure that all of the other lights in the room are off, if possible. Try to keep the amount of ambient light constant. It is also important that the probe and the light source are in the same position for all of the trials. The teacher could set the components up beforehand if possible. Students also need a basic understanding of the unit being used to measure light intensity. With most sensors it will probably be the lux. The lux (which is Latin for "light") is a unit of illumination equal to the illumination on a surface of one square meter in area on which there is a luminous flux of one lumen uniformly distributed.

Effective demonstrations can be made in front of the class using a transparency projector and large area polarized film. For example, by placing two films with the polarizing axis perpendicular to each on top of the projector glass, one can block out most of the light. Various orientations can be used.

Student Activity

To print out the Student Copy only, click here.

Materials

• a light probe
• computer or CBL (calculator based laboratory) with graphing calculator
• interface box (connects the probe with the computer) or CBL
• light source (such as a light bulb)
• large cardboard tube with slots to hold the filters
• 2 polarized filters

 Procedure

1. Gather the materials and set up as shown:
   
   
2. The probe and light source need to remain in the same position for all of the trials.
3. For each of the following conditions, repeat three times. Measure the amount of light detected through the tube without any filters. Record data.
   *For each trial, read the computer (CBL) to obtain the light intensity. Under "data" on the toolbar, click on "collect" when you have the filters set and are ready to collect data.
4. Place one filter into the slot and measure the amount of light detected through one filter. Record.
5. Turn that filter 90° and measure the amount of light detected. Record.
6. Return that first filter to its original position and place the other filter directly behind it. Measure the amount of light detected.
7. Turn the second filter 90° and measure the amount of light.
8. Record class data and analyze your results.
   
   

Data Sheet

To print out the Data Sheet only, click here.

Extensions

1. Students could also measure the effect of distance from the source on light intensity by moving the probe different distances from the source. They should design a data table to test their hypothesis.
2. Incorporating technology, students can use a graphing program such as Microsoft Excel to create their graph.
3. Students could research polarized lenses to discover exactly how they are produced and marketed.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

 

Assessment

Have students answer the following on a separate sheet of paper:

1. Which condition blocked the most light? Why?
   
   
2. What is the "control" situation in this experiment?
   
   
3. What are some factors that you tried to keep constant in this experiment?
   
   
4. Why was it important to repeat each condition three times?
   
   
5. Design a graph to show your results. *Be sure to place the independent variable on the x-axis.

Answers to Assessment

Sample Data should look something like this:

1. Two filters with the second one turned 90° blocked the most light because it was blocking both the horizontal and vertical light waves.
2. The control group is the situation in which there were no filters used.
3. The background lighting condition, the placement of the probe, light source (distance from the source)
4. The more trials you complete, the more valid your results will be. The effects of any bad data will be minimized when you compute an average.
5. Students should create a bar graph since the independent variable on the x-axis (the filter condition) is not continuous.