Physics: Polarization of Light

Understanding Physics: Polarization of Light

A polarizer does not choose light by brightness alone. Its material has a microscopic structure that responds more easily to electric fields in one direction than in the other. In many sheet polarizers, long aligned molecules absorb energy from fields pointing across them.

Fields pointing along the allowed direction pass with much less loss. This means the filter changes the wave itself, not just the amount of light reaching the eye. Real filters are not perfect.

They leak a little unwanted light and absorb some of the wanted light. That is why two crossed sunglasses lenses may look very dark rather than completely black.

The squared angle rule comes from two separate ideas. First, a wave can be split into components along chosen directions. The component that passes through the next filter becomes smaller as the axes turn apart.

Second, intensity measures energy flow, and wave energy depends on the square of the wave amplitude. A half-sized electric field therefore carries one quarter of the intensity. This is why brightness falls more quickly than a simple linear angle rule would predict.

Students should keep amplitude and intensity separate. Mixing them is one of the most common sources of errors in polarization calculations.

Reflection can create strong polarization without any filter. Light reflected from water, glass, roads, or snow often has a preferred vibration direction. At a particular incoming angle, called the Brewster angle, the reflected beam is almost completely polarized.

This explains why polarized sunglasses are especially useful near horizontal shiny surfaces. Their transmission axes are usually vertical, so they reduce much of the horizontally polarized glare. The effect is not the same as making the scene darker.

Useful light from objects can still reach the eye while reflected glare is reduced. Rotating the glasses while viewing a lake or a car window can make this change easy to observe.

Some materials affect different polarization directions by different amounts. In a transparent plastic object under stress, internal forces slightly change the material structure. Light components in two directions then travel at different speeds.

They leave the object out of step and can form colored patterns when viewed between polarizers. Engineers use this effect, called photoelasticity, to locate concentrated stress in models of bridges, tools, and machine parts. Liquid crystal displays use a related principle.

Electric signals rotate or control polarization inside each pixel, which changes how much light gets through the final polarizing layer. A useful classroom experiment uses three polarizers. Place the first and last at right angles, then put a third one between them at an intermediate angle.

Some light returns because the middle filter creates a new polarization direction in stages. This result shows that each filter acts on the light arriving at it, rather than simply adding darkness.