If the two systems of waves are separated the one from the other, viz., the horizontal from the perpendicular, they each form separately a ray of polarized light, and as Fresnel has remarked, common light is merely polarized light, having two planes of polarization at right angles to each other. To follow up the mechanical notion of the nature of polarized light, it is necessary to refer again to Woodward's card wave model (Fig. 323), and by separating the two cards one from the other it may be demonstrated how a wave of common light reduced to its skeleton or primary form is reducible into two waves of polarized light, or how the two cards placed together again in a transversal position form a ray of common light. (Fig. 324.)
Fig. 324.
No. 1. Common light, made up of the two waves of polarized light, Nos. 2 and 3.
The query with respect to the nature of polarized light being answered, it is necessary, in the next place, to consider how the separation of these transversal vibrations may be effected, and in fact to ask what optical arrangements are necessary to procure a beam of polarized light? Light may be polarized in four different ways—viz., by reflection, single refraction, double refraction, and by the tourmaline—viz., by absorption.
Polarization by Reflection, and by Single Refraction.
Fig. 325.
No. 1. a is the lime light. b. The condenser lenses. c. The beam of common light. Here the glass plates are removed.—No. 2. a. Lime light. b. The condenser lenses. c c. The bundle of plates of glass at an angle of 56° 45´. d is the ray of light polarized by reflection from the glass plates, c c, and e is the beam of polarized light by single refraction, having passed through the bundle of plates of glass, c c.