The phenomenon of double refraction was noticed, in the first instance, by Erasmus Bartholin, in Iceland-spar, a crystal the primary form of which is a rhombohedron; who perceived that the two images produced by this body were not in the same physical conditions.[100] It was also studied by Huyghens and Sir Isaac Newton, and to our countryman we owe the singular idea that a ray of light emerging from such a crystal has sides. This breaking up of the beam of light into two,—which is shown by looking through a pin-hole on a card through a crystal of Iceland spar, when two holes become visible, is due to the different states of tension in which the different layers constituting the crystal exist.
In thus separating the ray of light into two rays, the condition called polarisation has been produced, and by experiment we discover that the single ray has properties different from those of the compound or ordinary ray.
It is somewhat difficult to explain what is meant by, and what are the conditions of, polarised light. In the first instance let us see by what methods this peculiar state may be brought about.
If we reflect a ray of light from the surface of any body, fluid or solid, but not metallic, at an angle between 53° and 68° it undergoes what has been called plane polarisation. It may also be produced by the refraction of light from several refracting surfaces acting upon the pencil of light in succession; as by a bundle of plates of glass. Each surface polarises a portion of the pencil, and the number of plates necessary to polarise a whole beam depends upon the intensity of the beam and the angle of incidence. Thus, the light of a wax candle is wholly polarised by forty-seven plates of glass at an angle of 40° 41'; while at an angle of 79° 11' it is polarised by eight plates. Again, plane polarisation may be produced by the double refraction of crystals. Each of the two pencils is polarised, like light reflected from glass at an angle of 56° 45', but in opposite planes.
Non-scientific readers will still ask,—What is this mysterious condition of light which is produced by reflection and refraction at peculiar angles to the incident ray. It is one of the most difficult of problems to express in popular language. The conditions are, however, these:—
An ordinary ray of light will be reflected from a reflecting surface at whatever angle that surface may be placed in relation to the incident beam.
A polarised ray of light is not reflected in all positions of the reflecting surface.
An ordinary ray of light is freely transmitted through a transparent medium, as glass, in whatever position it may be placed relative to the source of light.
A polarised ray of light is not transmitted in all the positions of the permeable medium.
Supposing a plate of glass is presented at the angle 56° to a polarised ray, and the plane of incidence or reflexion is at right angles to the plane of polarisation of the ray, no light is reflected. If we turn the plate of glass round through 90°, when the plane of reflexion is parallel to that of polarisation the light is reflected. If we turn the plate round another 90°, so that the plane of reflexion and of polarisation are parallel to each other, again no light is reflected; and if we turn it through another 90° the reflection of the ray again takes place.