Question

Observe polarization of light due to a thin slit.

Hint:

You can use an LCD screen (from a phone, calculator, or monitor) as a source of already polarized light. If you look at an LCD screen through a single polarizing filter and rotate the filter, you will see the screen's brightness change, even becoming completely black at a certain angle.

Answer 1

Step 1: Understanding the Concept:
This question might be slightly misstated, as a single thin slit is primarily known for causing \textit{diffraction}, not significant polarization. Polarization is the phenomenon where light waves are restricted to oscillate in a single plane. While diffraction through a very narrow slit (comparable to the wavelength of light) can have minor polarization effects, the standard and most observable method to demonstrate polarization is by using polarizing filters (Polaroids). We will describe the observation using two polarizers, which is the standard demonstration.
Step 2: Apparatus Required:
- A source of unpolarized light (like a lamp or a torch). - Two polarizing filters (often called Polaroids). These can be obtained from a physics kit or by using lenses from 3D movie glasses or some types of sunglasses.
Step 3: Detailed Procedure:
1. Setup: Hold one polarizing filter (let's call it the Polarizer, P1) in front of the light source. Look at the light source through this filter. You will see that the light passes through, but its intensity is reduced by about half. The light emerging from P1 is now plane-polarized.
2. Using the Second Filter: Now, take the second filter (let's call it the Analyzer, P2) and place it in front of the first one, between your eye and P1.
3. Observation 1 (Parallel Axes): Initially, align P2 such that it allows the maximum amount of light to pass through. In this position, the transmission axes of the two polarizers are parallel.
4. Observation 2 (Rotating the Analyzer): Slowly rotate the Analyzer (P2) while keeping the Polarizer (P1) fixed. As you rotate P2, you will observe that the intensity of the transmitted light gradually decreases.
5. Observation 3 (Crossed Axes): When the Analyzer has been rotated by 90 degrees from the initial position, the intensity of the light becomes zero (or very close to it), and you will see darkness. In this position, the axes of the two polarizers are perpendicular, and they are said to be "crossed".
6. Further Rotation: If you continue to rotate P2, the light intensity will start to increase again, becoming maximum at 180 degrees.
Step 4: Conclusion:
This experiment demonstrates the polarization of light. The first filter polarizes the unpolarized light. The second filter (Analyzer) is used to observe this polarization by controlling the amount of polarized light that passes through it, according to Malus's Law (\(I = I_0 \cos^2\theta\)). The fact that the light can be blocked completely by the second filter proves that it is a transverse wave and has been polarized.