Question

Which of the following diagrams represent the variation of electric field vector with time for a circularly polarised light?

• A.
• B.
• C.
• D.

Answer 1

Answer: (A)

When two plane-polarised waves are superimposed, then under certain conditions, the resultant light vector rotates with a constant magnitude in a plane perpendicular to the direction of propagation. The tip of the vector traces a circle and the light is said to be circularly polarised.
To form circularly polarised light
$E_x=E_0\sin \omega t$
$E_y=E_0\cos \omega t=E_0\sin \left(\omega t+\frac{\pi }{2}\right)$
where $E_0$ is amplitude.
Resultant amplitude
$|\vec{E}{|}^2=E_0+E_0+2E_0\cdot E_0\cos \frac{\pi }{2}$
$|\vec{E}|=E_0\sqrt{2}=\text{ constant }$
Hence, the correct graph will be (a).