(a) What is meant by 'a beam of polarised light? (b) With the aid of well labelled diagrams, illustrate the action in of a polaroid spectacle on a beam of s...

(a) A beam of polarised light

Light is a transverse wave. In an ordinary (unpolarised) beam the electric-field vibrations occur in all planes perpendicular to the direction in which the light travels. A beam of polarised light is a beam in which these vibrations have been restricted to one single plane perpendicular to the direction of travel. In other words, the vibration of the wave takes place in only one plane (the plane of polarisation), so the light is said to be plane-polarised.

(b) Action of a polaroid spectacle on a beam of sunlight

Sunlight reaching the eye is unpolarised: its vibrations lie in every plane at right angles to the beam. A polaroid lens contains long parallel molecular chains that create a single transmission (polarising) axis. When sunlight strikes the polaroid:

• only the vibration parallel to the transmission axis is allowed through, and
• the vibrations in every other plane are absorbed.

The light that emerges is therefore plane-polarised and much reduced in intensity. This is why polaroid spectacles cut down the dazzling glare of sunlight (especially reflected glare, which is already partly polarised), so the wearer sees more comfortably. The action is illustrated below.

In the diagram, the incoming sunlight vibrates in all directions (shown by the double-headed arrows pointing every way). After passing through the polaroid, whose transmission axis is vertical, only the vertical component of vibration survives, so the beam reaching the eye is plane-polarised in the vertical plane and dimmer than the original sunlight.