Polarized and Unpolarized Light Explained in Simple Words

Ordinary light from the Sun or a bulb is unpolarized. Its electric field vibrates in all directions perpendicular to the direction of travel, changing randomly many times per second. Polarized light is light in which the electric field vibrates in just one plane.

Understanding polarization matters because it proves light is a transverse wave. It is also used every day in sunglasses, photography and liquid crystal displays. Polarization helps reduce glare and lets us control brightness.

Key Concepts

Plane of vibration: The plane containing the electric field and the direction of travel. In plane polarized light this plane stays fixed.

Unpolarized light: Equal vibrations in all perpendicular directions, with no fixed plane. Intensity through an ideal polarizer becomes half.

Polarizer and analyser: A polarizer passes only one vibration direction. An analyser is a second polarizer used to test polarization. When crossed at 90°, ideally no light passes.

How It Works

1. Start with unpolarized light travelling towards you. Imagine arrows pointing up, down, left, right and all angles in between, all shaking.
2. Place a Polaroid sheet. Its long molecules absorb one direction and pass the perpendicular direction. Only arrows aligned with its transmission axis get through.
3. The transmitted light now vibrates in one plane only. It is plane polarized and its intensity is half the original for ideal material.
4. Rotate a second sheet (analyser). Maximum light passes when axes align; minimum (near zero) when axes are crossed at 90°.

Worked Example

Unpolarized light of intensity 80 units falls on a polarizer and then an analyser. The analyser axis makes 30° with the polarizer axis. Find the final intensity using Malus's law.

Step 1: After the first polarizer, intensity I1 = 80/2 = 40 units.

Step 2: Malus's law: I = I1 cos² θ = 40 × cos²30°.

Step 3: cos30° = 0.866, cos²30° = 0.75. So I = 40 × 0.75 = 30 units.

The final beam has intensity 30 units and is polarized along the analyser axis.

Common Misconceptions

• Misconception: Polarized light travels in a straight line but unpolarized light wiggles in path. Correct: Both travel straight; only the vibration direction differs.
• Misconception: A polarizer makes light brighter. Correct: It always reduces intensity; half is lost at the first ideal polarizer.
• Misconception: Sound can be polarized the same way. Correct: Sound in air is longitudinal, so it cannot be polarized like light.

Real-World Applications

Polarized sunglasses block glare reflected from roads and water, which is partly polarized. Photographers use polarizing filters to darken skies and remove reflections. LCD screens use crossed polarizers with a liquid crystal layer to make bright and dark pixels.

To recap, unpolarized light vibrates in all planes, while polarized light vibrates in one. Polarizers select that plane and follow Malus's law.