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UW-Madison PHYSICS 208 - Lab 9 - Polarization

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Lab 9: Polarization Phy208 Spring 2008 Name_____________________________ Section___________ This sheet is the lab document your TA will use to score your lab. It is to be turned in at the end of lab. To receive full credit you must use complete sentences and explain your reasoning clearly. What’s this lab about? In this lab you investigate effects arising when using polarized light. There are four parts to the lab: PART A Use a polarizer to explore common examples of polarized light PART B Use a polariscope to understand how polarized light waves can be superposed or broken into components. PART C Create and investigate circular polarization. PART D Use the ideas of circular polarization to see how polarized light interacts with biological sugar molecules. Why are we doing this? Understanding polarized light is important applications such as microscopy, as well as in understanding the properties of electromagnetic waves.. What should I be thinking about before I start this lab? Suppose an electromagnetic wave is propagating through your area at the speed of light. If you could take a snapshot, you could freeze the electric and magnetic fields at a particular instant in time so that you could look at them. At each point in space around you, and at each instant in time, you would see electric and magnetic fields with these properties: i) ! r E and ! r B are always perpendicular to each other, and also to the wave propagation direction (such that ! r E "r B is in the propagation direction) ii) The magnitudes ! r E " E and ! r B " B are related as ! B xo, yo,zo,to( )= 1/c( )E xo, yo,zo,to( ). iii) ! r E and ! r B both oscillate in time with a frequency ! f = v /" (v is the propagation speed) iv) In takes a time T=1/f (the period) for the fields to complete one complete cycle at a fixed point in space, and a distance λ (the wavelength) to complete one complete cycle at a fixed time.2 A. Making and detecting polarized electromagnetic waves In a linearly polarized electromagnetic wave, the electric field varies in time and space, but it always lies in a plane of polarization. Since the associated magnetic field is always perpendicular to this, we can describe the polarization completely by describing the orientation of the electric field. Light doesn’t often look like this unless it is specially prepared. It is usually a superposition of all possible polarizations, which we call unpolarized light. A linear polarizer can be used to investigate the polarization state. It completely transmits linearly polarized light aligned with its transmission axis, and completely absorbs light linearly polarized perpendicular to its transmission axis. 1. Polarization by reflection. Light is partially polarized whenever it is reflected. You can check this by using the linear polarizer from the polarization kit on the lab table. The transmission axis is marked by raised lines on the black plastic disc that holds the polarizing material. i) First look through the polarizer directly at the black gooseneck lab table lamp. Is this light polarized? How did you tell? ii) Now aim the gooseneck lamp at about a 30˚to 45˚ angle (from the horizontal) at the lab bench, and look through the polarizer at the reflection of the bulb. Is this light polarized? In what direction is the plane of polarization? iii) Polaroid sunglasses operate on the same principle as your kit polarizer. In what direction is the transmission axis of Polaroid sunglasses? Explain. iv) Use your polarizer to look at your cell-phone display or a computer LCD monitor. Describe the light emitted by these.3 The polariscope Your polariscope has two polarizers (even though one is called an analyzer!): The bottom one sits directly over the (unpolarized) incandescent bulb, and is fixed in place. In a two-polarizer system, it is usually called the “polarizer”, since it produces linear polarized light from an unpolarized source. The top one can rotate. It is usually in-line with the bottom polarizer, but can also be swung to the side. In a two-polarizer system, it is usually called the “analyzer”, since it analyzes whether a sample placed above the bottom polarizer has altered the polarization. 1. Turn on your polariscope and look down into the top polarizer. Describe the intensity variations while rotating the top polarizer through 360˚. 2. Find the transmission axis of the bottom polarizer with the small polarizer (mounted in a black plastic ring) from your box. This one has its transmission axis marked with two ridges in the plastic. Explain your procedure and use a piece of masking tape to mark it on the rim of the polariscope. Find the transmission axis of the top polarizer and mark it also. 3. What angle is between the transmission axes of the two polarizers when there is: Maximum transmission? What about no transmission? Unpolaized light Polarizer Analyzer4 B. Transmission through a polarizer: In the previous section, you found that a linear polarizer will transmit a light wave whose electric field vector is parallel to the transmission axis, and blocks (absorbs) one with electric field vector perpendicular to that direction. For angles not 0º or 90º with respect to the transmission axis the polarizer transmits the component of the E-field along the transmission axis and absorbs the rest.. 1. Below are the electric fields of two linearly-polarized electromagnetic waves shown at various times throughout their period T. One is polarized along the x-axis and the other along the y-axis. Add these components together in each drawing and draw the electric field vector of the combined electromagnetic wave. y x y x y x y x y x t=0 t=T/6 t=2T/6 y x t=3T/6 t=4T/6 t=5T/6 y x t=6T/6 x-component y-component What angle does the plane of polarization make with the y-axis?5 2. Now suppose a wave with electric field amplitude Eo is linearly polarized at an angle of 30˚ with respect to the transmission axis of the analyzer. What are the amplitudes of the components parallel and perpendicular to the transmission axis? 3. Write down the amplitude of the E-field of the wave transmitted by the analyzer. What is the polarization angle of this transmitted wave with respect to the analyzer’s transmission axis? 4. What is the ratio


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UW-Madison PHYSICS 208 - Lab 9 - Polarization

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