of incidence less than 90". This particular angle is called 

 the critical angle and its value depends on the two media com- 

 prising the system. When the critical angle has been passed, 

 the incident wave, instead of refracting into the second medium, 

 rebounds into the first medium. This phenomenon is called total 

 reflection and the angle which the reflected wave makes with the 

 normal to the boundary of the two media is called the angle of 

 reflection. 



We now have a whole new family of problems available for 



investigation and some of these will be studied in future sections, 



2 



7. Refraction of light by a vertical glass cylinder 



A geometrical optics solution has been obtained by Chin- 

 mayanandura (1918) for the intensity of illumination along an 

 internal ray for any angle of incidence, 9, Using monochromatic 

 light of wave length 5^5 ^ 10 ^ cm. emanating from a horizontal 

 collimator slit, and incident on a glass cylinder of radius 

 0.0184 cm., Chinmayanandum made the photograph shown in fig^^re 

 5, and from the available data he constructed the diagram in 

 figure 6. 



To study the analogous problem in the ripple tank, it was 

 necessary to set a plastic disc in the water. The disc, 8 inches 

 in diameter and 1 inch high, was submerged in the tank which was 

 filled with water one and one-eighth inches deep. The photographic 

 result is shown in figure 7 and the projection of the wave front 

 after refraction is shown in figure 8. 



2. Chinmayanandum, T.K. (1918): Diffraction of light by an 



obliquely held cylinder. Physical Review , vol. 12, pp. 314-324. 



17 



