Polarization by Scattering and Absorption Easy Explanation

Light can become polarized in two common natural ways: by scattering from tiny particles and by selective absorption in special materials. Scattering explains why the sky looks polarized when viewed through Polaroid sunglasses. Selective absorption is how Polaroid sheets and tourmaline crystals work.

These ideas matter because they connect daily observations to wave theory. They also give simple, cheap ways to produce and control polarized light without complex instruments.

Key Concepts

Scattering: When sunlight hits air molecules or tiny dust, it gets redirected. The scattered light vibrates more strongly in the plane perpendicular to the original direction.

Selective absorption (dichroism): Some crystals absorb one vibration direction much more than the other. The transmitted light is therefore polarized.

Polaroid: A plastic sheet with long aligned polymer molecules, often coated with iodine. It absorbs one component and passes the other.

How It Works

1. Scattering: Unpolarized sunlight travels through air. Molecules act like tiny antennas, re-radiating light sideways. An antenna cannot radiate along its own length, so sideways light lacks one vibration component and becomes partially polarized. At 90° to the Sun, polarization is strongest.
2. Selective absorption: Unpolarized light enters a Polaroid. The aligned molecules strongly absorb the electric field parallel to them. The perpendicular component passes with little loss.
3. After either process, test with an analyser. Rotating it changes brightness, proving the light is polarized.

Worked Example

A student looks at the sky at 90° from the Sun through a Polaroid and rotates it. At one position intensity is maximum 60 units; at 90° rotation it drops to 15 units. Estimate the degree of polarization.

Step 1: For partially polarized light, degree of polarization P = (Imax − Imin) / (Imax + Imin).

Step 2: Insert values: P = (60 − 15) / (60 + 15) = 45 / 75 = 0.60.

Step 3: P = 60 percent. The scattered light is 60 percent polarized, not fully, because air contains larger particles and multiple scattering.

If it were fully polarized, Imin would be zero and P would be 1.

Common Misconceptions

• Misconception: Sky light is always fully polarized. Correct: It is only partially polarized and strongest at 90° from the Sun; it is weak near the Sun.
• Misconception: Polaroid works by reflection. Correct: Polaroid works by selective absorption inside the sheet, not by surface reflection.
• Misconception: Any transparent sheet polarizes light. Correct: Only materials with aligned absorbing molecules or specific crystals show strong dichroism.

Real-World Applications

Bees and some other insects navigate using polarized sky light. Photographers use scattering polarization to deepen blue skies. Polaroid sunglasses and camera filters rely on selective absorption to cut glare and improve contrast in a cheap, thin form.

In short, scattering polarizes sky light sideways, and selective absorption in Polaroid selects one vibration. Both are simple proofs of light's transverse nature.