When light travelling in air strikes the surface of a body of
water at a shallow angle, it is split into two components; the
refracted ray, which penetrates the water, and the reflected ray, which
bounces from the surface. What may be less obvious is that
the two components are of different polarisation; the reflected ray
having its electric field parallel to the surface and being said to be horizontally
, and the refracted ray having its e-field
perpendicular to the surface, and being said to be vertically
. This means that we can separate rays
reflected from the surface from rays coming from under the surface
using a polarising filter, i.e., a rotatable filter made from material
that blocks the passage of polarised light of a particular orientation.
The effect of a polarising filter is
illustrated by the three photographs below (which were taken in
Constantine Bay, near Padstow, Cornwall, UK).
Vertically polarised light is blocked and what lies under the
water cannot be seen.
Horizontally polarised light is blocked, eliminating surface
glare and allowing us to see through the water.
Note that polaroid (or polarised, or anti-glare) sunglasses are
designed to facilitate driving by suppressing horizontally polarised
reflections from road surfaces. They work equally well in
suppressing reflections from the surface of water, and are therefore
extremely useful for the purpose of assessing underwater conditions
sunglasses, pick up two pairs of alleged polarisers and look through
two lenses at once. If they are genuine, the combination will
go dark when one is rotated at 90° relative to the other.
Here light from a flash has been bounced from an air-water boundary to
illuminate the buildings beyond. In this case the light striking the
surface is split into two components of opposite polarisation, the
horizontally polarised component being reflected and the vertically
polarised component entering the water. A polarising filter fitted to
the camera can be rotated to enhance either the surface reflections or
the view beneath the water.
© David W Knight 2012