Explain polarisation by scattering.

The sunlight gets scattered by the molecules present in the atmosphere. Under the influence of the electric field of the incident wave, the electrons in the molecules acquire components of motion in the two mutually perpendicular directions. The accelerating charges are the source of energy radiation. The dots stand for polarisation perpendicular to the plane of the figure and the double arrows show polarisation in the plane of the figure. Consider an observer looking at 90 0 to the direction of the sun. Clearly, charges accelerating parallel to double arrows do not radiate energy towards the observer since their acceleration does not have a transverse component. The radiation scattered by the molecule is therefore represented by dots. It is polarised perpendicular to the plane of the figure as shown in the figure below.

State the law of Malus.

According to the law of Malus, the intensity of the polarised light transmitted through the analyser I varies as the square of the cosine of the angle θ between the plane of transmission of the analyser and the plane of the polariser. If I o is the intensity of light coming out of the polariser, this law can be written as: I = I o cos 2 θ

Show using a proper diagram how unpolarized light can be linearly polarised by reflection from a transparent glass surface.

When an unpolarized ray of light strikes upon a transparent glass surface, the reflected light gets polarised. But when the reflected and refracted light are perpendicular to each other, the ray of light gets completely plane polarised. Thus, if i p and r be the angle of incidence and refraction, when reflected and refracted light is perpendicular to each other respectively, then: i p + 90 ° + r = 180 ° Using Snell's law: μ = sin ( i p ) sin ( r ) Using the result from above, μ = sin ( i p ) sin ( 90 ° - i p ) μ = sin ( i p ) cos ( i p ) = tan ( i p ) This is the equation for Brewster's law. The angle of incidence i p is called Brewster's angle. Thus, when an unpolarized light is an incident on a transparent glass surface at Brewster angle, the reflected light is plane-polarised light. The diagram for the same is given below: