162 ELEMENTARY GENERAL SCIENCE CHAP. 



SOME LAWS RELATING TO LIGHT. 



Light Travels in Straight Lines. That light rays travel in 

 straight lines can be at once shown by examining the path of these 

 waves as they pass through a hole in a shutter of a darkened 

 room. Though, as has been learnt, the light-waves are not 

 themselves visible, yet the path of the light becomes apparent 

 because the minute particles of dust in the air are rendered 

 luminous by the vibrations of the ether being absorbed by them. 

 The path of the light can thus be followed, and is seen to be in 

 a straight line. We can infer this from several everyday 

 experiences. We cannot see round a corner ; if light travelled 

 in lines that were sometimes bent (we are speaking of a 



FIG. 71. To illustrate that Light travels in Straight Lines. 



uniform medium), there is no reason why we should not. Or, 

 again, every one knows that it is only necessary to put a small 

 obstacle in the path of the light from a luminous body to com- 

 pletely shut out our view of it. The light from the setting sun, 

 when the sky is cloudy, is often seen to travel in straight lines. 



EXPT. 158. Take three cards and make a small hole in each 

 with a fine needle. Fix the cards on to wooden blocks so that 

 all the holes are at the same height and in a straight line. 

 Place a lighted candle or a lamp in front of the card, and look 

 through the third (Fig. 71). As long as the holes are in a 

 straight line you can see the light from the candle shining 



