Question

What is plane-polarised light ? How the ordinary light, partially polarised light and totally polarised light are distinguished with the help of a polaroid ?

Hint:

The key to distinguishing the types of light is to look for the minimum intensity when rotating the analyser. \begin{itemize} \item No change in intensity \(\implies\) Unpolarised. \item Intensity becomes zero \(\implies\) Completely Polarised. \item Intensity has a non-zero minimum \(\implies\) Partially Polarised. \end{itemize}

Answer 1

1. Plane-Polarised Light:
An ordinary light wave (unpolarised light) consists of electric field vectors oscillating in all possible directions perpendicular to the direction of wave propagation. Plane-polarised light is a light wave in which the electric field oscillations are restricted to a single plane containing the direction of propagation. This plane is called the plane of polarisation.
2. Distinguishing between types of light using a Polaroid:
A polaroid (also called an analyser) is a device that only allows light with a specific polarisation orientation to pass through it. By passing the incident light through a polaroid and rotating the polaroid by \(360^\circ\), we can observe the intensity of the transmitted light and distinguish between the three types:
\begin{itemize} \item Case 1: Ordinary (Unpolarised) Light: \begin{itemize} \item Process: When unpolarised light is passed through a polaroid, the transmitted intensity is always half of the incident intensity (\(I = I_0/2\)). \item Observation: As the polaroid is rotated, the intensity of the transmitted light remains constant. There is no variation in brightness. \end{itemize} \vspace{0.2cm} \item Case 2: Totally (Plane) Polarised Light: \begin{itemize} \item Process: When plane-polarised light is passed through a polaroid, the transmitted intensity follows Malus's Law: \(I = I_{max} \cos^2\theta\), where \(\theta\) is the angle between the polarisation plane of the light and the pass-axis of the polaroid. \item Observation: As the polaroid is rotated, the intensity of the transmitted light varies from a maximum value to zero. The light will be completely extinguished (intensity becomes zero) at two positions in a full \(360^\circ\) rotation (when \(\theta = 90^\circ\) and \(\theta = 270^\circ\)). \end{itemize} \vspace{0.2cm} \item Case 3: Partially Polarised Light: \begin{itemize} \item Process: Partially polarised light is a mixture of unpolarised and plane-polarised light. \item Observation: As the polaroid is rotated, the intensity of the transmitted light varies between a maximum value and a non-zero minimum value. Unlike totally polarised light, the intensity never becomes zero. \end{itemize} \end{itemize}