Polarization of Visible Light Purpose Problem 1: to investigate the polarization of common light sources; Problem 2 and 3: to verify Malus’ Law for transmission of polarized light; Problem 4: to observe polarization of light by reflection and to verify Brewster’s Law. Note: Problems 1 and 3 only need Purpose, Apparatus, Data and Conclusion sections. In sentences, write your observations (answering all the questions) in Data section and summarize the results in the Conclusion section. Problem 2 and 4 need all sections: Purpose, Apparatus, Data, Calculation, Precision Analysis, Conclusion and Discussion. Introduction and Theory Light is a transverse electromagnetic wave: it consists of electric and magnetic fields oscillating perpendicular to the direction of propagation, as well as perpendicular to each other. If the electric fields oscillate only in one direction, then the light is said to be linearly polarized or simply polarized, and the direction of the electric fields is called the direction of polarization. In this lab, we will produce and examine the polarization of visible light with polarizers made from Polaroid sheets. These sheets consist of long chain molecules of polyvinyl alcohol which have been caused to line up in one direction by stretching. The sheets are then dyed with iodine. The iodine atoms attach themselves to the polyvinyl alcohol molecules, forming parallel conducting chains. These chains absorb the component of light with electric field oriented along the chains, and allow the light with electric field perpendicular to the chains to pass through. The direction perpendicular to the chains is called the transmission axis, therefore: The Transmission Axis The transmission axis is a fixed direction on the Polaroid sheet. Light is transmitted through the Polaroid if its electric field is parallel to the transmission axis. Light is absorbed if its electric field is perpendicular to the transmission axis. Polaroid sheets were invented by Edwin H. Land in 1938. He formed the Polaroid Corporation to market them. He later developed the instant photo system which is commonly referred to by the Polaroid name.
1225 Polarization of Visible Light ­ 1 Problem 1 Transmission through One Polarizer Apparatus A polarizer in a holder, an incandescent light bulb. Data 1. Look at a light bulb through the polarizer. Does the light appear less intense through the sheet? Rotate the polarizer. Does the light bulb’s intensity vary? 2. Look through the polarizer at skylight about 90° away from the sun. Does the intensity of the skylight vary when you rotate the polarizer? Note whether clouds are present. On a cloudy day, observe the reflection of the light bulb on a tabletop instead of the skylight. Conclusions According to your observations, is light from a light bulb polarized? Does the skylight (or the reflection of light on your table) appear to be polarized? Problem 2 Transmission of Polarized Light through a Polarizer, Test of Malus’ Law Apparatus Draw a labelled diagram of the apparatus. See Fig 1. light source photo cell to light meter Polarizer A Polarizer B track Fig. 1
Data In this problem, we are going to investigate how polarized light transmits through a polarizer. The first polarizer (A) produces polarized light. The second polarizer (B) examines the incident light. For this reason, the second polarizer is also called the analyzer. The amount of light that can pass through the analyzer depends on the polarization of the incident light relative to the transmission axis of the analyzer, or the angle between the transmission axes of the two polarizers q. If the light passing through Polarizer A is perpendicular to Polarizer B, or q = 90°, no light can pass through. 1225 Polarization of Visible Light ­ 2 1. Rotate Polarizer A so that 0 is at the mark. See Fig. 2. 0
Fig. 2 2. I1 is the intensity of light that incidents on Polarizer B. Measure it by removing Polarizer B and pressing the cardboard tube against Polarizer A. Repeat a few times to get the uncertainty δI1. Using the light meter
· Put the cardboard cup of the light meter against the centre of the polarizer;
· If the light is strong, you may have to use the second setting of the light meter, 19990/20000. If you do, multiply your readings by 10. You should switch to the most sensitive setting (1999/2000) when light is weaker. 3. Mount Polarizer B. Look through Polarizer B while rotating it a full cycle. This gives you a qualitative idea of how the intensity changes as the angle q changes. 4. Rotate Polarizer B to the 0 mark. The two polarizers should be parallel, or q = 0°. Look through Polarizer B to ensure maximum light is transmitted. 5. To check that two polarizers are parallel, rotate Polarizer B to 40° and to -40° (320°) and compare the transmission intensities measured by the light meter. Make sure they differ by no more than 5%. If not, you should adjust Polarizer A. For example, if I2(40°) = 118 Lux, and I2(-40°) = 102 Lux, you should leave Polarizer B at -40° and rotate Polarizer A slightly, so that the light meter reads 110 Lux (the average). After you adjust Polarizer A, double check that the intensities at ±40° agree within 5%. This step ensures the two polarizers are indeed parallel when Polarizer B is at 0°. 6. Rotate Polarizer B from q = 0º to q = + 90° and then from q = 0º to q = -90° at intervals of 10° and measure the transmission intensity I2 as a function of q. Record q, I2 and their uncertainties in a data table. Also list I2 Ave (q), the average of I2(q) and I2(-q), from q = 0º to q = + 90°. Calculations Graph the average transmission intensity I2 Ave versus cos 2q for 0 £ q £ 90°. Create another table before graphing, listing each quantity to be plotted and its uncertainty. How do you find d (cos 2q)? 1225 Polarization of Visible Light ­ 3 Conclusions What can you conclude about the relationship between the transmitted light intensity and the angle q? Is your conclusion consistent with the assumption that only the component of the electric field that is parallel to the transmission axis passes through the polarizer? (Hint: the intensity of light is proportional to the square of the strength of the E­field.) Discussions You should be able to express the relationship you got by I 2 = b I 1 cos 2 q , where b is called the transmission coefficient. Calculate b along with its uncertainties from the slope of your graph and I1. Is b bigger or smaller than 1? Should it be? Problem 3 Changing the Polarization with a Polarizer Apparatus Similar to Fig. 1, but a third polarizer, Polarizer C, is also used. Data 1. Set the apparatus as in Fig 1. Rotate Polarizer B until you see minimum light. What must be the angle (or angles) between the two transmission axes? 2. Keep Polarizer A and B fixed at one of those angles (so light cannot pass through them). Now we will see, by adding a third polarizer in between the first two polarizers, we can make light pass through. Mount the third polarizer onto Polarizer A so it is between the first two polarizers and call it Polarizer C. Rotate Polarizer C while looking through all three polarizers. Can you see light at certain angles? In a data table, record the angles of all three polarizers each time you see maximum light. There should be 4 entries in your data table. Conclusions Explain why light can pass through three polarizers but cannot pass through without the middle polarizer. Use vector concepts and draw a diagram to illustrate. Theoretically, what are the directions of the three transmission axes? Does your experimental result verify your theory? Problem 4 Polarization by Reflection Apparatus Beaker with water, one polarizer in its holder, a light bulb on the stand. Data 1. Look at the reflection of the light bulb on the surface of water in a beaker. Find the angle between the incident light and the surface normal of the water, when the reflected light is most polarized. This is Brewster’s angle, qp. Measure it as accurately as you can. Show your method with a diagram. 2. Calculate the theoretical value of Brewster’s angle qp REF (see your textbook), and use this as a reference value. Complete Calculations, Uncertainty Analysis, Conclusions and Discussion sections.
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