BRIGHTNESS, OR INTENSITY, OF LIGHT 



153 



We call the distinct shadow, which receives no light at all, 

 the umbra; the indistinct part surrounding the umbra is 

 called the penumbra. 

 Fig. 143 gives the 

 illustration : 



FIG. 143. 



The shadow cast by an object that is illuminated by 

 the sun's surface consists of umbra and penumbra. 



Let us suppose that 

 the moon, in the cone- 



f>Qr>Prl crmpA tr tVi^ rio-ht 



of the earth, is entirely 



cut off from sunlight. To an observer on that side of the earth the 

 moon would be entirely eclipsed by the earth's shadow (cf. 3). To 

 an observer on the moon the sun would be entirely eclipsed by the 

 earth. But if the moon were in the penumbra of the earth's shadow, 

 an observer on the moon would see the earth as a black body covering 

 part of the sun's disc, but not all. If an observer could stand just at 

 the tip of the umbra, and facing the earth, the sun's face would be just 

 exactly covered by the earth. 



170. Brightness, or Intensity, of Light. We all know 

 that the distance of an object from a source of light affects 

 the amount of light received by the object, or the intensity 

 of the light. Now, if one boy holds his book twice as far 

 away from a reading lamp as another boy does, he receives, 

 not ]/2 as much light, but J4 as much light as the other. 

 A third boy, sitting three times as far away as the first, 

 receives only 1 / 9 as much light, and so on. 



The reason for this is shown by Fig. 

 144. Two square cards, 1 inch and 2 

 inches on each edge, respectively, are 

 placed in vertical positions on the same 

 FlG side of the lamp. The lamp should be 



An object twice as far from the lamp very small, so that its light shall come 



as another gets only \ as p n . , / /. a -, /rv\ 



much light. from practically one point (cf. 169). 



