XI 



LIGHT 169 



refraction, and the ray is said to be refracted. We will de- 

 scribe how this can be shown experimentally in the case of light- 

 waves, but what holds good of waves of this particular length 

 holds true of all ether- waves, as well as of any other kind of 

 radiation. 



EXPT. 165. Procure a rectangular metal box, such as a cigar- 

 ette box, and put a wooden or metal scale on the bottom. In 

 a darkened room let sunlight fall slantwise against the edge. 

 The side of the box throws a shadow which reaches, say, to C 

 (Fig. 79), which, since light travels in straight lines in the 

 same medium, is a continuation of the rays of sunlight A B. 

 Without disturbing anything fill the box with water. The 

 shadow no longer reaches to C, but only as far as D. Clearly 

 the light-waves have been bent or refracted out of their 

 original course. Notice that NN' is the normal, and that the 

 light travelling from the comparatively rare air into the com- 

 paratively dense water is refracted towards the normal. 



This experiment illustrates the general fact of the deviation 

 experienced by a ray of light in passing from one medium to 

 another of different density. The laws of refraction can be 

 determined by another simple experiment. 



EXPT. 166. Upon a piece of board, ABCD (Fig. 80), place 



sheet of paper, and upon the paper put a piece of fairly 



thick glass EF with parallel sides (a thick piece of glass from a 



box of weights, a paper-weight, or a number of slips of micro- 



no. 80. Method of Determining Kefraction of Light. 



scope glass will do very well). Rule along the edges of the 

 glass with a finely-pointed pencil. Place two pins a, 6 as 

 shown in the illustration, and then, looking through the glass 

 from the other side, stick in the pins c, d so that all four 

 appear in a straight line. 



Now take away the glass and pins and join the pin-holes on 

 the paper as shown in Fig. 81. Draw the normal ebf, and the 



