Download Polarization I-Advanced Physics-Lab Report and more Exercises Advanced Physics in PDF only on Docsity!
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 = E 1 sin(ωt - kz+δ) Ex = E 2 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 = (E 1 sin(ωt - kz + δ)) / (E 2 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.
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.
Fig. 2. Plane polarized light
We can see in figure that Ex and Ey components of electric field have constant angle between them so their resultant is in single plane and does not oscillate so it is called plane polarized
light.
To make polarized light from un-polarized light, it is passed through a polarizer. A polarizer is a
material which can pass light of only specific electric field direction. Polarizer has a free
direction. When light of random polarization strike polarizer, polarizer pass only those waves
from light beam which have electric field parallel or having one component parallel to free axis
of polarizer. All other waves or components which are not parallel to free axis will be blocked.
So only specific electric field orientation light is passed called plane polarized light. As polarizer block all electric field or components of electric field of un-polarized light waves which are
perpendicular to free axis of polarizer, so intensity of polarized light is less than that of un-
polarized light. In fact is one half intensity of un-polarized light.
If a polarized light is passed through a polarizer, then intensity of light passed through polarizer
depends on angle of polarization of light and axis of polarizer. If direction of polarization of light
and axis of polarizer are in same direction then maximum intensity pass through polarizer. If this
angle π/2, then no intensity is passed. Mathematically
I = I 0 cos^2 θ
This is called law of Malus. This means if two polarizer are at π/2 angle to each other then no
light will be passed.
Light can also be polarized through reflection process.
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.
- 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 90o^ from previous position and observing that minimum intensity passed in this new position, which showed 90o angle between polarizer and light’s polarization direction.
- 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 0o, 180o, 360o^ intensity has maximum value while for angles 90o, 270o^ 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.
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.
- 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
- 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.
as shown.
Fig. 8, intensity through two polarizers when first is fixed
- QWP fixed and polarizer rotating :- In this case our desire was to fix QWP at 45^0 but it was not possible due to scale given on stand. Let first case QWP placed before the polarizer. If QWP is placed the polarizer and QWP is fixed and polarizer is rotated then QWP will change polarized light into circularly or elliptically polarized light which has two components of intensity (E field) as Ix and Iy. For rotating polarizer, intensity passed through it is given by Malus law as ( ) ( ) Where θ is the angle between Ix and free axis of polarizer. This formula is written for first quadrant but can be used for full revolution of polarizer. If θ=45^0 then I=Ix( or Iy). But if θ has values other than 45^0 then intensity has some variations given as
In second case polarizer which was placed before QWP is rotated while QWP was kept fixed at 45^0 (almost). In this case again QWP did nothing with light coming from polarizer and intensity pattern showing effect of only polarizer. Again normalized intensity plot against angle of rotation of polarizer for 45^0 of QWP is shown.
Fig. Normalized intensity against rotation of polarizer placed before QWP (45^0 )
- QWP rotating and polarizer fixed:- In first case polarizer was placed before the QWP and QWP was rotated. Polarizer was placed at 45^0 angle with polarization direction of light. Polarizer blocked light according Malus Law. Again QWP did nothing with intensity but changed only phase which is not measurable by detector. So graph shows no variation in intensities and almost is constant.
Fig. Normalized intensity plot for rotating QWP shows no variations. In case two, QWP was rotated and placed before polarizer which was at 45^0. In this case QWP was rotating so it was producing light of circularly polarized at 45^0 , 135^0 ,225^0 and 315^0 angles of fast axis of QWP and polarization direction of light. On angles 0^0 ,90^0 ,180^0 ,270 0 and 360^0 QWP had no effect on polarization of light as polarized light had direction parallel either to fast axis or slow axis of QWP. So at those angles maximum intensity was observed according to Malus law. ( ) ( )
