MICROSCOPY 



91 



phase. If the two waves vibrate in opposite directions, they cancel each 

 other, and interference resuhs (see Fig. 8-6). 



A + A= 2A 



-A 



A-A = 



Fig. 8-6. Reinforcement and interference. In the upper set of curves, 

 the waves are in phase, and their ampHtudes are additive. In the lower 

 set of curves, the waves are exactly out of phase, so they cancel each 

 other. 



Imagine a beam of Hght of one wavelength approaching a barrier with 

 two parallel slits, as in Fig. 8-7. Each slit acts as a new source of light. 

 Point on the screen is the same distance from a as from h, so light 

 arriving from a and from h are in phase and reinforce each other, pro- 

 ducing a bright spot on the screen. Point 1 on the screen is one wave- 



