90 MICROSCOPY 



Although it cannot be seen from Fig. 8-5, the frequency of vibration 

 of the hght beam remains constant, the speed of propagation changing 

 because the wavelength changes. The degree of change in wavelength is 

 dependent upon the wavelength itself. For example, shorter wavelengths 

 are retarded more upon passing from air to glass. For this reason, the 

 refractive index of a material is expressed relative to a certain wavelength 

 of light, usually the sodium line at 5893^4(589.3 mju,). 



Reflected 



Fig. 8-5. Refraction at an air-glass interface. 



When light passes from glass to air, the same kind of bending will 

 occur, but in the opposite direction. Thus a pair of parallel surfaces 

 causes a jog in the light path, but it is not apparent because the entering 

 and emerging beams move in the same direction. A lens, however, has 

 opposite surfaces inclined at angles to each other, so that the emerging 

 beam may have quite a different direction from the entering beam. In 

 a microscope, every plane in the light path where hght passes from one 

 medium to any other must be given careful consideration. 



Diffraction: According to Huygen's principle, every point on an ad- 

 vancing wavefront acts as a new source of light. This leads to an apparent 

 bending of light around corners, known as diffraction. 



If two parallel wavetrains happen to be vibrating in the same direc- 

 tion, their amplitudes are additive, and the two waves are said to be in 



