Polarization-Physics-Lab Report, Exercises of Physics

Download Polarization-Physics-Lab Report and more Exercises Physics in PDF only on Docsity! To study nature of polarization of laser light Submitted to: Dr. Asloob Ahmed Mudassar Submitted by: Yasir Ali M.Phil. Physics DPAM PIEAS docsity.com Any electromagnetic radiation consists of two components, the electric and the magnetic, which are mutually perpendicular. The plane, in which one component lies say electric component, is known as the plane of oscillation for that wave. During excitation and de-excitation an atom emits EM radiation. During single such event, the EM radiation emitted has fixed plane of polarization. But when a large number of such events combine, the plane of radiation is random for all such events. Such light is called randomly polarized or un-polarized. In un-polarized light, electric field of light continuously changes its electric field. As in EM radiation electric field and magnetic field are perpendicular to each other, so magnetic field also continuously changes its field. But we always can resolve E-field into its horizontal and vertical components. As light is also an electromagnetic radiation so we discuss a particular case. In light electric field continuously changes its direction so its components, when direction of propagation is along z- axis, are gives as Ey = E1 sin(ωt - kz+δ) Ex = E2 sin(ωt - kz) This means that one component is delayed by phase difference δ which is random for un- polarized light. The angle which this resultant electric field makes with the X axis can be expressed as, tan θ = Ez / Ey = (E1 sin(ωt - kz + δ)) / (E2 sin(ωt - kz)) From angle Ѳ we can define different categories of light depending on orientation of electric field. Un-polarized light:- For random phase difference δ, angle Ѳ between two components of light is also random so light is called randomly polarized or un-polarized. As we can see that plane of electric field oscillates around the direction of propagation of light. This is called un- polarized light. Fig.1 Unpolarized light Plane polarized light:- If phase difference between x-axis component and y-component of electric field is such that δ=0 or δ = π then angle Ѳ between the two components is constant and light in this case is known as linearly polarized light. In linearly of plane polarized light electric field is confined only in one plane and is perpendicular to direction of propagation of light. docsity.com Elliptically polarized light is produced by passing linearly polarized light through quarter wave plate when angle between polarization direction and fast axis of quarter wave plate is other than 45 o . In case of elliptically polarized light, intensity does not change. Only decomposing a single vector into its rectangular components with some phase difference does not change magnitude on vector. Similar is case for elliptically polarized light. We can also produce linearly polarized light from circularly or elliptically polarized light by passing it through quarter wave plate. Experiment:- Apparatus used in our experiment is, two polarizers, two quarter wave plates, photo detector to measure intensity, He-Ne laser. Objective:- To study nature of polarization of different kinds through polarizer and quarter wave plates. Procedure:- Steps in which the experiment was performed are following. 1. Direction of polarization:- First of all we found direction of polarization of laser light. This was done by placing a polarizer in the path of laser beam and rotating it smoothly. At certain point maximum intensity of light passed through polarizer. At that point direction of polarized light and free axis of polarizer were aligned. So this is polarization direction of light. This was verified by rotating polarizer by 90 o from previous position and observing that minimum intensity passed in this new position, which showed 90 o angle between polarizer and light’s polarization direction. 2. Intensity and first polarizer:- For observing effect of polarizer’s angle with light’s polarization using first polarizer. We started from zero angle and noted intensity through detector. Then we changed angle between polarizer and polarization direction of light in small steps and noted intensity. From we can observe that intensity vary with angle. For angles 0 o , 180 o , 360 o intensity has maximum value while for angles 90 o , 270 o intensity has minimum value shown in plot. Fig. 3. Intensity through second polarizer Intensity and second polarizer:- Then we used another polarizer and observed variation on intensity due to rotation of polarizer. Plot of intensity of transmitted polarized light through polarizer for different angles between polarizer and direction of polarization of light is given. docsity.com Fig. 4. Intensity through second polarizer 3. Quarter wave plate rotating:-. No intensity variations are observed when QWP is rotated and no polarized was oresent. 4. Intensity through two polarizers:- Next we placed two polarizers in front of laser beam and measured intensity passed through them. Here we had two cases 1. Polarizer 1 (placed before second) rotating and polarizer 2 fixed:- To study the effect of presence of one polarizer on other polarizer, we fixed second polarizer at angle 45 0 and rotated first polarizer for 360 0 angle. As first polarizer rotates by 360 0 angle so intensity passed through it will be different given by Malus law. For 0 0 , 180 0 light passed through will be unaffected while for angles 90 0 and 270 0 zero intensity will be passed. For all other angles, light from laser will be changed to two components that is , Ex and Ey with corresponding intensity of Ix and Iy respectively. Fig. 5. Showing polarized light through polarizer. docsity.com These two components now will pass through another polarizer which is at nearly 45 0 angle to original direction of polarized light. Now according to Malus law, intensity passed through second polarizer is ( ) ( ) Fig. 6. Two components of polarized light through another polarizer. Intensity pattern observed are Fig. 7Intensity through two polarizers 2. Polarizer 2 rotating and polarizer 1 fixed:- This part is same as above, the difference is only that now polarizer 1 is kept at 45 0 (almost) to direction of polarized incident beam and polarizer 2 is rotated for 360 0 angle. In this case polarizer 1 affected intensity on light by Malus law and also destroyed its polarization and give x-component and y-component of light which after passing through polarizer 2 again through Malus law and equation given above give intensity as shown. If polarizer 1 was exactly at 45 0 angle then intensity through both polarizers would be constant, but as we could not fixed that polarizer at 45 0 exactly so some small variations observed docsity.com