110 LIGHT 



so long as the medium is constant ; whenever the direc- 

 tion of a ray is changed, it is sharply broken at a point 

 and passes on in a straight line until again changed. 



128. The Ether. Light waves travel long distances, 

 as from the sun and far more distant stars, through 

 space which we know to be rarer than any vacuum that 

 man can make. Clearly no air is needed to carry these 

 waves. Yet we must suppose that some medium is neces- 

 sary, even though it may be very rare; therefore we 

 speak of this medium just as if it were known to exist, 

 and call it the ether ( 93). The ether is supposed to 

 fill all space, even entering the pores of solid matter. 

 Light waves are then assumed to be vibrations of the 

 ether, as sound waves are vibrations of the air. 



129. Speed of Light Waves. Through space, light 

 waves travel about 186,000 miles per second. This speed 

 is so great that for all distances through which we can 

 see on earth, the waves travel instantly. A ray of light 

 would pass entirely around the earth seven times in 

 one second; and rays from the sun, 93,000,000 miles 

 away, reach the earth in a little over eight minutes. 



As with sound waves, the speed of light waves varies 

 in different media. In general, rays travel faster through 

 a rare than through a dense medium. 



130. The Passage of Light Waves Some substances 



allow light waves to pass freely through them; glass, 

 air, and water are examples. We can see through them 

 clearly, and they are said to be transparent. 



Other substances allow light waves to pass through, 

 but scatter them in different directions ; ground glass 



