8 min readConcept sheetSection A MCQ (1 mark) or objective (2 marks, two parts); Section B very short answer (2 marks) on why polarisation shows that light is a transverse wave

Polarisation of light and Polaroids

In plane polarised light the vibrations lie in one plane. Only transverse waves can be polarised, so polarisation proves that light is transverse.

Unpolarised lightLight whose vibrations point in all directions at right angles to the ray, as in sunlight or light from a bulb

Plane polarised lightLight whose vibrations are confined to one plane containing the direction of travel

PolaroidA sheet of tiny crystals (quinine iodosulphate) lined up in a plastic film; it passes vibrations along one direction, its transmission axis, and absorbs the rest

Crossed PolaroidsTransmission axes at 90° to each other: no light comes through

Activity: a rope wave through two slits

1
Cut a narrow vertical slit in each of two cardboard sheets. Hold them parallel, a little apart.
2
Pass a long rope through both slits. Tie one end to a hook and hold the other end.
3
Shake your hand up and down, sideways, and in all directions in between.
4
Observation: only the up and down vibrations pass the first slit, and they also pass the second vertical slit.
5
Turn the second slit so that it is horizontal. Observation: no wave gets past it.
6
Result: the first slit has polarised the wave. This works only because a rope wave is transverse; a push along the rope (longitudinal) would pass any slit.

Polarisation of light

Repeat the activity with light, using two Polaroid sheets in place of the slits. Light from a bulb is unpolarised: its vibrations point in all directions at right angles to the ray. The first Polaroid, the polariser, lets through only the vibrations along its transmission axis. The light that comes out is plane polarised, and it is less bright. Look at it through a second Polaroid, the analyser, and turn it slowly. The light is brightest when the two axes are parallel and disappears when they are at 90° (crossed). This can happen only if the vibrations of light are at right angles to its direction of travel. So polarisation proves that light is a transverse wave. Sound in air is longitudinal, so it cannot be polarised.

Unpolarised vs plane polarised light

Unpolarised lightVibrations in all directions at right angles to the ray. Brightness does not change when you turn a single Polaroid in front of it. Examples: sunlight, a bulb, a candle.

Plane polarised lightVibrations in one plane only. Brightness changes as you turn an analyser, and becomes zero when the analyser is crossed. Example: light that has passed through a Polaroid.

How to draw polarisation by two Polaroids

Draw a source S on the left and a ray going to the right.
On the ray before the first Polaroid, draw short double-headed arrows in several directions (or arrows and dots); label it unpolarised light.
Draw the first Polaroid as a thin upright rectangle with vertical lines on it; label it polariser P₁ (axis vertical).
After P₁, draw only vertical double-headed arrows; label it plane polarised light.
Draw the second Polaroid P₂ (analyser). Show light passing when its axis is vertical, and no light beyond it when its axis is horizontal; label this crossed Polaroids.

Uses of Polaroids

Sunglasses with Polaroid lenses cut glare from water, wet roads and glass.
Camera filters remove reflections and bring out details hidden by glare.
LCD screens of calculators and watches use Polaroid sheets to form their display.
3D films are watched with glasses whose two lenses pass light polarised in different directions.
Polarimeters, used for quality control in the sugar industry, measure sugar in solutions with polarised light.

In real life

Look at an LCD calculator screen through Polaroid sunglasses and tilt your head: the display goes dark at one angle, because both use polarised light. Fishermen wear Polaroid glasses to cut the glare from the sea.

Exam angle

2 marks: “How does polarisation show that light is a transverse wave?” Light from a polariser is cut off completely when an analyser is turned to 90°. This is possible only if the vibrations are at right angles to the direction of travel. Longitudinal waves, such as sound, cannot be polarised.

2 marks: “Distinguish between unpolarised and plane polarised light.” Unpolarised: vibrations in all directions at right angles to the ray; brightness does not change when a Polaroid in front of it is turned. Plane polarised: vibrations in one plane; brightness changes with an analyser and is zero when it is crossed.

1 marks: “Unpolarised light falls on two Polaroids. What should the angle between their transmission axes be to block the light completely?” 90° (crossed Polaroids).

Remember it

Pass a Polaroid, pick a plane. Crossed Polaroids (90°) give darkness.