Polarization Polarization 2010 Table of Content Section Page Background………………………………….3 Experiment 1………………………………...4 Experiment 2………………………………...6 Analysis…………………………………….10 -2- Areej Al-Jarb Polarization 2010 BACKGROUND Light is a form of electromagnetic radiation, just like Radio waves, television waves, radar, microwaves, infrared waves, X-rays, and Gamma rays. One of the characteristic of light is that light is broken up into discreet units. They are actually bundles of energy which we call photons. Just like in a stream of water, it is actually water molecules (H2O) which are moving down the river. In a beam of light, it is actually photons of light which are moving along at the speed of light. Light, like all electromagnetic radiation, exhibit the properties of wavelength and frequency. So we know that light acts like a wave. As the light goes from left to right it actually follows the wavy line that goes up and down as it goes toward the right. So we can see that the photon of light can vibrate up and down as it goes toward the right. Now each photon is independent from the other photons. So we could have some photons vibrate up and down, others vibrate in other directions. That is exactly what happens. Each photon vibrates in it's own plane, or it's own direction. What a polaroid lens does is to let through the light that vibrates only in the proper direction. The picture shows the first lens (in both Experiment 1 &2), as only letting through the light that vibrates up and down. All the other light is stopped. Now, it's what we do with the second lens that determine the outcome of the experiment. If we have the second lens oriented in the same direction as the first lens, (as in Experiment 1) then only the light that vibrates up and down will pass through both lenses. If on the other hand, the second lens is oriented as in Experiment 2, (letting only the light that vibrates right and left) then no light will reach your eyes. -3- Areej Al-Jarb Polarization 2010 Experiment 1 Linear polarization with polarizing lenses THEORY A polarizer only allows light which is vibrating in a particular plane to pass through it. This plane forms the “axis” of polarization. Unpolarized light vibrates in all planes perpendicular to the direction of propagation. If unpolarized light is incident upon an “ideal” polarizer, only half will be transmitted through the polarizer. Since in reality no polarizer is “ideal”, less than half the light will be transmitted. The transmitted light is polarized in one plane. If this polarized light is incident upon a second polarizer, the axis of which is oriented such that it is perpendicular to the plane of polarization of the incident light, no light will be transmitted through the second polarizer. However, if the second polarizer is oriented at an angle so that it is not perpendicular to the first polarizer, there will be some component of the electric field of the polarized light that lies in the same direction as the axis of the second polarizer, thus some light will be transmitted through the second polarizer (see the bottom figure). -4- Areej Al-Jarb Polarization 2010 The component, E, of the polarized electric field, Eo, is found by: E = E0 cos Ø Since the intensity of the light varies as the square of the electric field, the light intensity transmitted through the second filter is given by: I = I0 cos2 Ø where Io is the intensity of the light passing through the first filter and Ø is the angle between the polarization axes of the two filters. Consider the two extreme cases illustrated by this equation: • If Ø is zero, the second polarizer is aligned with the first polarizer, and the value of cos2Øis one. Thus the intensity transmitted by the second filter is equal to the light intensity that passes through the first filter. This case will allow maximum intensity to pass through. • If Ø is 90º, the second polarizer is oriented perpendicular to the plane of polarization of the first filter, and the cos2 (90) gives zero. Thus no light is transmitted through the second filter. This case will allow minimum intensity to pass through. • These results assume that the only absorption of light is due to polarizer effects. In fact most polarizing films are not clear and thus there is also some absorption of light due to the coloring of the polaroid filters. -5- Areej Al-Jarb Polarization 2010 EQUIPMENT Laser Polarizing filters (2) Photometer OBJECTIVE To show that the intensity of the transmitted light through two polarizers depends on the square of the cosine of the angle between the axes of the two polarizers. PROCEDURE 1. Project the laser beam into the display tank and insert a polarizer into the path of the beam so that it can be rotated in the plane perpendicular to the beam. 2. Next place a second polarizer (analyzer) between the first polarizer and the display tank. Rotate the analyzer so it make 0o with polarizer's axis. 3. Place a detector (photometer) in the path of the transmitted beam and record the intensity (Io) 4. Rotate the analyzer so it make an angle Ø with polarizer, record this angle. 5. From the detector record the intensity. 6. Continue to rotate the analyzer and record the intensity for each orientation. 7. Plot a graph to determine the relationship between the light intensity and the square of the cosine of the angle -6- Areej Al-Jarb Polarization 2010 Experiment 2: POLARIMETER The polarimeter is used for the determination of the concentration of optically active substances. For this the sodium light with a wavelength of 589 nm is linearly polarized by means of a polarizer. This light then passes through the solution to be investigated and is observed through an analyzer. Optically active substances rotate the polarization plane. By measuring the rotational angle by means of the analyzer, the concentration of the solution can now be calculated. 1 Polarimeter a) eyepiece with magnifying lens for b) scales and vernier c) adjustment screw for turning the analyzer d) sample chamber with cover e) polarizer f) sodium lamp with cover g) mains switch 2 Sodium lamp 3 Round cuvette 100 mm 4 Round cuvette 200 mm . Technical data Polarimeter Measuring range: Scale division: Reading precision: 0 ... 180 ° 1° 0.05 ° (vernier) sodium lamps Wavelength: 589 nm Round cuvette 100 Length: 100 mm Diameter: 10 mm Round cuvette 200 Length: 200 mm Diameter: 10 mm -7- Areej Al-Jarb Polarization 2010 THEORY Once the light goes through the first polarizer lens (just like a polarid lens) only the light that vibrates up and down get through. Now the light enters the tube that is filled with the active substance. When it goes through the solution, the light begins to twist. The plane of light changes so that after the light comes out of the tube, it is now vibrating in another direction, not up and down, but a different direction. It is the job of the second polarizer lens (analyzer) to determine how much the light has twisted or rotated. This second polarizer is rotated by the scientist until the light disappears. Then the angle is noted and recorded. So a Polarimeter actually measures how much the light has been rotated by a specific substance. To test another substance, the scientist can replace one tube with another tube that contains a different substance. -8- Areej Al-Jarb Polarization 2010 OBJECTIVE To measure the coefficient of specific rotation for the sugar solution. EQUIPMENT –polarimeter –sugar solution of different concentration (5% , 10% , 15% , 20% , 25% , 30). SETUP 1.Optical zeroing (calibration) Switch on the sodium lamp and wait a short time for it to warm up. Set the analyzer using the adjustment screw to approx. 90°. Set the eyepiece to such a value that a clearly delimitated bright circle can be seen. Turn the analyzer with the adjustment screw to 0° so that the circle is uniformly lit (dark), i.e. so that no central line is visible. When the analyzer is turned about the zero position the central line and the two edge areas become light or dark. For this reason deviations from the zero position can very easily be observed. Reading off the angle by means of the vernier scale through the corresponding magnifying lens in the eyepiece edge should now show 0°. 2.Filling a round cuvette Hold the round cuvette vertical so that the thicker part is at the top. Take off the upper screw ring; remove the internal cap, sealing ring and glass disc. Fill the round cuvette with the liquid to be investigated with as few bubbles as possible. Replace the glass disc, sealing ring and internal cap and tighten with the screw ring. 3.Determination of the angle of rotation Place a round cuvette with the liquid to be investigated into the sample chamber in such a way that the thick part is on top. If necessary collect any air bubbles in the area of the thick part so that they do not interfere with the path of light. Turn the analyzer with the adjustment screw in such a way that the observed circle is again uniformly lit. Read the angle of rotation α from the scale by means of the vernier scale or work out the average of the two readings to increase the precision. -9- Areej Al-Jarb Polarization 2010 4.Determination of the concentration of a solution The following applies: s= with concentration : c angle of rotation : θ ml/g ° length of the light path : l dm specific rotation: s ml ° /g dm PROCEDUR 1. Fill the tube with the first concentration (5%) and maintain the situation of darkness as the zero order. Record the angle of rotation. 2. Repeat the previous step with the other concentration. Tabulate your results . 3. Plot a graph between the concentration c (x-axis) and the rotation angle θ (yaxis) which will give a straight line. 4. Find the slop of the line then substitute in the relation : s= - 10 - Areej Al-Jarb Polarization 2010 NAME -------------------------------------- ANALYSIS Experiment 1 I0 = --------------------------- I - 11 - I/I0 φ Areej Al-Jarb Cos2φ Polarization 2010 Experiment 2 C l= 2 dm θ 0% 5% 10% 15% 20% 25% unknown Slope = --------------------------------s = --------------------------------------- - 12 - Areej Al-Jarb