(a) What is plane polarised light? Two polaroid sare placed at `90^(@)` to each other and the transmitted intensity is zero. What happens when one more polaroid is placed between these two, bisecting the angle between them? How will the intensity of transmitted light vary on further rotating the third polaroid?
(b) If a light beam shows no intensity variation when transmitted through a polaroid which is rotated, does it mean that the light is un-polarised? Explain briefly.

Explanation

### Step-by-Step Solution

#### Part (a): What is Plane Polarised Light?

1. **Definition of Plane Polarised Light**:
- Plane polarised light is light in which the electric field oscillates in a single plane. This is achieved by filtering unpolarised light (which has electric field vectors oscillating in multiple planes) through a polaroid, allowing only the components of the electric field that align with the polaroid's axis to pass through.

2. **Setup with Two Polaroids**:
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