is parallel to the z-axis and the incident light travels in the 

 xy plane, then the slit itself may be regarded as a single point 

 on the incident wave front, and according to Huygens ' principle, 

 the slit becomes the origin of a spherical wavelet, and the only 

 source of wave activity beyond the slit (fig. 20). 



In the case of a plane wave front which advances along 

 parallel lines, the wave front upon passing through the slit be- 

 comes spherical and the direction along which it advances becomes 

 radial, and therefore diverging. 



In the subject of physical optics, many such phenomena due to 

 the spreading of light waves which pass through small apertures 

 are known and are studied under the general name of diffraction. 

 In all such cases, the openings in question bear about the same 

 relation to the wave length of light as the openings in the analo- 

 gous acoustical phenomena bear to the wave length of sound. 



Consider the emitted light to be lying in a plane which passes 



through the source Q and is perpendicular to the edges of a slit. 



This plane is the xz plane (fig. 21). Let the x coordinates of 



the slit be x-, and Xp. If the point P , where the intensity is 



to be calculated, is in the geometrical shadow of one of the 



screens which bounds the slit on either side, then x, and Xp are 



both positive or both negative. But if the line connecting Q with 



Pq passes through the open slit, then the signs of x,^ and Xp are 



opposite. 



The intensity of illumination (J) at some point P a distance 



■^ o 



pQ from the plane of the slit was calculated anri ±s given by the 

 expression 



47 



