Laws of Reflexion.
Plane Mirror.—A parallel beam is used. The spectrometer-circle is adjusted with the zero division opposite to the radiator. The platform index is turned to zero, and a plane reflector placed on a previously marked diameter at right angles to the index. The receiver is placed, say, at 60°. The platform carrying the mirror is slowly rotated (electric radiation being at the same time produced by interrupting the key), till the receiver suddenly responds. It will now be found that the platform index points to 30°, midway between the radiator and the receiver.
Curved Mirror.—A cylindrical metallic mirror, with a radius of 25 cm., is placed on the platform, with its principal axis coinciding with the platform index. When the radiator is placed at a distance of 25 cm. from the mirror, the source of radiation would be at the axis of the cylinder. The reflected image will now be formed at an equal distance. The receiver mounted on the radial arm (at a distance of 25 cm. from the centre) is placed at a given angle; the platform is rotated till the receiver responds. The index will now be found to bisect the angle included between the radiator and the receiver.
Refraction.
Deviation of the Electric Ray by a Prism.—An isosceles right-angled prism is made of sulphur or ebonite. Parallel beam is used. For showing deviation by refraction one of the acute angles is interposed on the path of the beam.
Total Reflexion.—An interesting experiment on total reflexion is shown in the following way:——The receiver is placed opposite to the radiator, and the prism interposed with one of its equal faces at right angles to the direction of the ray. The receiver will remain unaffected. The critical angle of ebonite being considerably less than 45°, the rays undergo total reflexion. On turning the receiver through 90° it responds to the totally reflected ray.
Opacity due to Multiple Refraction and Reflexion.—An experiment analogous to the opacity of powdered glass to light is shown by filling a long trough with irregular-shaped pieces of pitch, and interposing it between the radiator and the receiver. The electric ray is unable to pass through the heterogeneous media, owing to the multiplicity of refractions and reflexions, and the receiver remains unaffected. But on restoring partial homogeneity by pouring in kerosene, which has about the same refractive index as pitch, the radiation is easily transmitted.
