674 Prof. Nihal Karan Sethi on the Diffraction 



77- /\ 

 And since the ano-le of incidence at Q is 7, — ?r, 



VQ' = ^asm~dA. 



From these it is evident that while AT decreases as A is 

 increased, VQ' increases. It is true that while YQ/ does 

 represent the actual intensity of the reflected pencil, AT 

 does not do so; for a fraction of this light is reflected at 

 each surface of the cylinder and is lost to us. But a con- 

 sideration of the Fresnel coefficients of refraction, viz. 



[sin 2/ sin 2r~| 2 , [~ sin 2z sin 2?' ~| 2 



sin 2 (1 + ?■)_, a Lsin 2 (l+r)cos 2 (l + r)J ' 

 for components polarized in the plane of incidence and at 

 right angles to it, will show that this will modify AT very 

 slightly at first, and will begin to appreciably affect it at 

 considerable angles only. In the case considered, the value 

 of these factors does not fall below about '9 even for i — S0°, 

 and only comes down to *5 for z = 81 c '4 nearly. The only 

 effect of this is to shift the region of equality of intensities 



Flo: 



slightly towards the origin, but the main features are still 

 preserved. This correction has, however, been applied in 

 drawing the curve of fig, 2, which shows graphically how 

 the value of the intensity varies with A in the resulting 



