60 



PROGRESS IN MICROSCOPY 



0-55//. The amount of reflected light is shown by the expression: 



Reflected luminous flux 

 R 



Incident luminous flux 



R is the reflectance of the surface. Tf the glass surface is 

 coated its reflectance is least when the wave-length Aq = 055 fi 

 and highest when at the blue and red ends of the spectrum, the surface 



Fig. 1.76. Glass surface coated by a thin film. 



then being purplish red if observed by reflection. Let us consider 

 flint-glass refraction index A'^ = 1-62. Uncoated, its surface has 

 reflectance R = 0-056 at normal incidence. Coated with a thin film 

 of thickness ne = A/4 when Aq = 0- 55 ju the reflectance of the glass 

 optical plate is 001 when A^ = 055 ju. A stiU lower reflectance in 

 relation to the wave-length can be obtained by the use of achromatic 

 thin film (Turner). Instead of one, two thin films are now deposited 

 on the glass surface (Fig. 1.77). The first thin film has an optical 



V rr 'i<<i >'. Vr tt <-t, ,,t',Y,/ft <//,'/< 



,^2 / 



Glass N 



"2 \ --^0^^ 



Fig. 1.77. Glass surface coated by two thin llhns. 



thickness n^ei = A/4, that of the second one //o^o = A/2 (between the 

 first and the glass). Figure 1.78 shows the reflectance distribution of 

 A^-index glass in relation when A'^=l-62, /;, 1 4 and //. = 20. 

 Curve 1 shows the R changes when the glass surface is coated with 

 one thin film. Curve 2 shows the variations of R when there are 

 two achromatic thin films on the glass. At wave-length Xq =055// 



