Plane polarisation

A polarised wave oscillates in only one plane (e.g only up and down if vertically polarised), only transverse waves can be polarised.

Below is a diagram which shows the effect of vertically polarised and horizontally polarised waves passing through a block with vertical slits, which acts as a vertically polarising filter.

(a) The vertically polarised wave passes through the filter without a problem. 

(b) The horizontally polarised wave cannot pass through the filter as it blocks waves which are not in the vertical plane.

Polarised sunglasses are an application of polarisation. They reduce glare by blocking partially polarised light reflected from water and tarmac, as they only allow oscillations in the plane of the filter to pass through, making it easier to see.