Plane-polarised light
Ordinary light vibrates in all planes perpendicular to its direction of travel. A polariser blocks every plane but one, so the light that emerges vibrates in just a single plane — this is plane-polarised light.

What's happening

Press Start to send light down the polarimeter, one stage at a time.

Readout

Solution rotation+30°
Analyser axis0°
Light reaching eye0%
Analyser not aligned
Drag the analyser disc to rotate it. When its axis matches the rotated plane, the light passes through and the angle read off is the optical rotation.
+30° (d)

Unpolarised vs plane-polarised

Light from an ordinary source vibrates in every plane at right angles to its path. Plane-polarised light vibrates in only one plane. A polariser (such as Polaroid) transmits one plane and absorbs the rest.

Optically active substances

A substance is optically active if it rotates the plane of polarisation. This happens when its molecules are chiral (non-superimposable on their mirror image), e.g. a single enantiomer of a compound with a chiral carbon.

Clockwise or anticlockwise

One enantiomer rotates the plane clockwise (dextrorotatory, +); its mirror image rotates it equally anticlockwise (laevorotatory, −). A 50:50 racemic mixture gives no net rotation.

The analyser measures it

The analyser is a second polariser, rotated until the light passes through brightest. The angle it must be turned through is the observed rotation, which depends on the substance, concentration and path length.