CHAPTER XII 

 THE SUN'S (HPT OP LIGHT 



Light. The sun is not only the source of almost all 

 the heat of the earth but also of its light. We have devel- 

 oped artificial self-luminous bodies such as candles, lamps, 

 electric lights, but none of these compares with the light 

 given by the sun. The stars also furnish a little light. 



Light is just as essential to life as heat is. If plants or 

 animals are where light is entirely excluded, they begin 

 to sicken and die. If they are placed where it is very cold, 

 they freeze and die. Although the sun gives both heat and 

 light, yet these two are not inseparable. We feel the heat 

 given out by boiling water but there is no light, and we 

 see the light of the moon but there is no appreciable heat. 

 We usually say that we feel heat but cannot see it and see 

 light but cannot feel it. 



Direction of Light Movement. Experiment 100. Point 

 the pinhole end of a camera obscura or pinhole camera (this con- 

 sists of two telescoping boxes, the 

 larger having a pinhole at the end 

 and the smaller a ground glass plate 

 (Figure 97)) at some object and move 

 the ground glass plate back and forth 

 until a sharp image of the object is FIGURE 97 



formed. Sketch on a piece of paper 



the object and the image, showing the direction in which you think 

 the rays of light must have traveled through the pinhole to form 

 the image. 



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