112 



LIGHT 



in which the rays from some part of this luminous body 

 may fall, as the portions ace 1 and bdd' in Fig. 81. 

 A dark central portion, as ac'd'b in Fig. 81, receives no 

 rays at all ; this is called the umbra of the shadow. The 

 lighter portion, which receives some (but not all) of the 

 rays from fe, is called the penumbra. 



Darkness, on earth, is always due to shadows. Even 

 the darkness of night is caused by our passing into 

 the shadow of the earth. In daytime we do not find 

 it dark within shadows of trees, buildings, or other 

 objects ; this is because sunlight is reflected into these 

 spaces from air particles and other bodies all around 

 them. 



132. Reflection. When a light wave strikes any sur- 

 face and is turned off from it, the wave is said to be 

 reflected. This is important. It is due to reflection 

 that objects can be illuminated ; that is, objects that are 



6 , not luminous can be 



" 



seen by means of the 

 waves which fall 

 upon them and are 

 reflected to the eye. 



Experiment 88. 



Stand before a mirror, 

 as at a (Fig. 82), hold- 

 ing a lighted candle in 

 front of y.ou. Rays from 



the candle strike the mirror at right angles, at &, and come straight 

 back to you. Now move the flame toy. Find a point e from which 

 your eye sees the reflection of the flame at b. Draw lines fb and 

 eb ; measure the angles dbf and cbe. How do they compare ? Take 

 a new pointy and repeat. 



FIG. 82 



