Contemporary Advances in Physics, XVI The Classical Theory of Light, Second Part

E A S U R E M E N T of wave-lengths is the subject which we shall now consider. So entitled, the topic seems unpromising, as some d r y exercise in mensuration; b u t in t r u t h it is distinguished for b e a u t y and variety, and implicated with the whole of modern physics. This is not measurement of the lengths of palpable objects, as pieces of lumber or cloth, which are laid alongside of a yardstick or clamped in t h e jaws of a gauge. In optics, the methods of measuring wavelengths are t h e methods of proving t h a t waves exist, therefore of testing the u n d u l a t o r y theory of light. One could not reasonably ask for evidence of light-waves more convincing t h a n the concord of the values obtained for the wave-length say of sodium yellow light, by all the diverse i n s t r u m e n t s which act by causing interference or diffraction: N e w t o n ' s tapering film of air between a lens and a plate, Fraunhofer's grid of iron wires, the tilted mirrors of Fresnel, Michelson's echelon, and all the m a n y gratings and interferometers continually in use in laboratories and classrooms. Wave-lengths of X - r a y s are comp u t e d from the diffraction-patterns imposed on X - r a y beams by intercepting crystals, a n d these p a t t e r n s were t h e evidence which showed some fifteen years ago t h a t the rays are of the n a t u r e of undulations, though it could not disprove t h a t in some paradoxical w a y t h e y are also of t h e n a t u r e of corpuscles. F r o m similar diffractionp a t t e r n s imposed by crystals on electron-streams it follows t h a t these also are p a r t l y of the n a t u r e of waves, and again the p a t t e r n s have supplied the values of the wave-lengths.