a question in one of the GATE previous paper is as such

a plane EM wave travelling along +z- direction, has its electric field given by Ex = 2 cos (*t) and Ey = 2 cos (*t+90° ).
the wave is

options given are
a). linearly polarised
b) right circularly polarised
c) left circularly polarised
d) elliptically polarised


I request you the answer. i would also like to know how to calculate a wave (as given in this problem) its polarisation?

 

Well let's start off.

A plane polarised wave has components in one plane only. this wave has components in two plane, the xz plane and the yz plane.
So it is not plane polarised.

Examining the components further we note that they are of equal amplitude and at right angles. Two cosine waves of equal amplitude at right angles combine to form a circle. They would form an ellipse of the amplitudes were different.
So the wave is circularly not elliptically polarised.

To determine the handedness of the circularly polarised wave consider what happens when we combine the components.

We have a difficulty here are there are two conventions:

http://en.wikipedia.org/wiki/Circular_polarization

We are told to look in the direction of travel, i.e. source to receiver which make this polarisation left handed, since the Y is pi/2 in advance of the x.