REFRACTION 



117 



6. How fast do light waves travel ? How long time are the 

 sun's rays in coming to earth? How does the speed of light 

 waves vary with the density of the medium ? 



7. Define a transparent substance ; a translucent substance ; 

 an opaque body. State examples of each. What may become of 

 the waves that fall upon an opaque body ? 



8. What is a shadow? How is a shadow caused ? Name the 

 two portions of a shadow, explaining the difference. 



9. Why is it dark at night? Why is it not entirely dark in 

 the shade in daytime ? Why is it not dark on cloudy days ? 



10. What is meant by reflection? State the Law of Reflec- 

 tion. Why do light waves from the same source make different 

 impressions upon the eye when reflected from different things? 



11. Explain the difference between reflection from smooth and 

 from rough surfaces. What sort of surfaces reflect rays so as to 

 form images ? Give examples. 



12. How are waves reflected from curved surfaces? What is 

 the focus of a curved reflector ? Explain the use of reflectors in 

 obtaining a powerful illumination. 



SECTION II 

 REFRACTION 



136. Examples of Refraction. "We have learned that 

 light waves travel usually in straight 

 lines ( 127), and also that the direction 

 of these lines may be changed whenever 

 a ray strikes a point and is reflected 

 (132). In another way the direction 

 of a wave may be changed, when it 

 passes from one medium to another that 

 is denser or rarer. Such a change of 

 direction is called refraction, and the wave is said to 

 be refracted. 



FIG. 87 



