Change Of Polarization At Metallic Surface
Change of polarization at metallic surface: The transmitted light can be converted into p polarized light completely by allowing it to pass through large number of interfaces. One can use a large number of glass plates separated by a small distances, in air, to form a series of air- glass interface as shown in fig 5.
fig..(1) Polarization of transmitted light by pile of glass plates
In this case, if the light is falling at Brewster angle then at each interface there will be the reflected light containing only s component and so from the transmitted light the s component will vanish slowly and leaving only the p component. One can define the degree of polarization P for the transmitted component as
-----------------------------------(1)
I s and I p are the intensities of s and p component in the transmitted light respectively. If there are m glass plates placed at Brewster's angle then the degree of polarization is given by
-----------------------------------(2)
Where, m is the number of glass plates used and n is the refractive index of the glass plate. From eq 2 one can say that if m is very large then degree of polarization for the transmitted light will be approaching to one, thereby the transmitted light will be completely p polarized. Therefore the pile of glass plates at Brewster angle, gives only p polarized light in the transmitted direction.
The pile of such glass plate can be termed as polarizer, which has a property to deliver the polarized light.
3. Malus Law
We have seen above that a pile of glass plate acts as a polarizer. Suppose that Two such pile of plates are used as shown in fig 6.When the planes of incidence for both the sets are parallel (i.e angle of incidences are at Brewster angle) the output from the second plate will be p polarized light. . Now suppose that the second pile of plate is rotated about the axis X-X' as shown such that the plane of incidence changes for the second set of glass plates.. You will observe that the light intensity will start decreasing and finally when the second plate is rotated by 90 ° , the output light is zero because for this position the electric field is perpendicular to the plane of incidence and the light will be reflected completely from it. On further rotation, the light output will be restore at 180 ° . The cycle will continue from 180 ° -360 ° . Thus in one complete revolution of the second pile the light will under go maxima and minima twice.
fig..(2)
We can say that first pile of glass plate is acting as a polarizer and the second pile is analysisng it and hence can be termed as analyzer. The orientation of the analyzer which allows the polarized light to pass completely is termed as the pass plane of the analyzer.
Let us under stand quantitatively the variation of light intensity as a function of the rotation angle of analyzer.