LIGHT 221 



Light travels in straight lines through a transparent 

 medium of uniform density. 



Sources of Light. Much of the light we enjoy comes 

 from the sun ; but since the sun is hidden from our view 

 for one half the time of each year, man has devised 

 numerous means of producing artificial light. Whale- 

 oil lamps, pine knots, candles, kerosene lamps, gas lamps, 

 and electricity have been used for the purpose of giving 

 light. The light of flames is due to little particles of 

 carbon which have been heated to incandescence by the 

 combustion of gases from heated oil or wax. Most light 

 sources are quite hot. 



Luminous Bodies. We cannot see light itself, but 

 we can see the bodies which emit the light. Such bodies, 

 as the sun, the lighted candle, the arc light, are called 

 luminous bodies. Every point in a luminous body sends 

 out rays of light in all directions. Most of the objects 

 we see are non-luminous. We see them by the light which 

 they receive from some other source and then reflect to 

 our eye. If a non-luminous object reflects no light, it 

 is invisible. The moon is a non-luminous body and 

 receives its light from the sun. 



Transparent, Translucent, and Opaque Bodies. - 

 From observation we have learned that light passes 

 through many substances, such as air, glass, water, and 

 mica. Substances through which objects may be dis- 

 tinctly seen are said to be transparent. Substances which 

 allow some of the light to pass, but through which ob- 

 jects cannot be seen distinctly are called translucent. 

 Parchment, oiled paper, and ground glass are examples 

 of translucent substances. Substances such as stone 

 and wood through which light does not pass are called 

 opaque substances. 



