LIGHT AND VISION 157 



9. In the tropics, twilight is much shorter than in temperate regions, 

 and near the poles, it is much longer. Why? (What part of the earth 

 turns away from the sun most rapidly?) 



10. How does seeing the candle flame differ from seeing the candle? 



11. Why is a snowflake or ground glass white, when water and glass 

 are transparent? 



12. Why does a piece of white cloth or a light soil turn darker when 

 wet? 



13. How does frosted or roughened glass prevent our seeing through it? 



14. Why are we unable to see out of the window after the lamps are 

 lighted? 



15. How does painting the walls of cellars and other dark places white 

 increase the light in them? 



16. Put a coin in a moderately deep basin and stand so that the coin 

 is just hidden from view by the edge of the vessel. Let a classmate slowly 

 pour water into the basin without disturbing the coin. How does this 

 affect your seeing it? 



17. Explain how this effect was produced, 



18. Why does a pole projecting from the water appear to be bent at 

 the surface of the water? 



19. Put a spoon in a glass of water and, looking down on it, place your 

 finger on the outside of the glass at the point where the spoon seems 

 to touch it. How does this compare with the real position of the spoon? 



