AEROGRAPHER'S MATE 3 & 2 



INCIDENT LIGHT 



REFLECTED LIGHT 



209.17 



Figure 12-16, — Terms used to describe the re- 

 flection of light. 



as the incident light. The path of the light 

 reflected from the surface forms an angle 

 exactly equal to the one formed by its path 

 in reaching the medium. This conforms to the 

 law of reflection which states: The angle of 

 incidence is equal to the angle of reflection. 



Reflection from a smooth- surfaced object 

 presents few problems. It is a different matter, 

 however, when a rough surface reflects light. 

 The law of reflection still holds; but because 

 the surface is rough, the angle of incidence 

 will be different for each ray of light. The 

 reflected light will be scattered in all directions 

 as shown in figure 12-17, 



This form of reflected light is called 

 irregular or diffused light, 



REFRACTION 



The change of direction which occurs when 

 a ray of light passes at an oblique angle (less 

 than 90°) from one transparent substance into 

 another of different density is called refraction. 



Refraction is due to the fact that light travels 

 at various speeds in different transparent 

 substances; that is, substances of different 

 density. It travels faster in less dense 

 substances. 



Refraction, or change of direction, always 

 follows a simple rule. When the light ray passes 

 from one transparent substance into another of 

 greater density, refraction is toward the 

 normal. (In this rule, the normal means a 

 line perpendicular to the surface of the medium 



SPECULAR 



INCIDENT LIGHT REFLECTED LIGHT SCATTERED 



DIFFUSED 



209.18 



Figure 12- 17, — Reflected light. (A) Regular 

 (specular); (B) irregular (diffused). 



at the point of entrance of the light ray.) In 

 passing from one transparent substance into 

 another of lesser density, refraction is away 

 from the normal. 



Consider a ray of light entering a denser 

 medium at an angle of 90° (perpendicular), such 

 as from air to water as illustrated in figure 

 12-18. The speed of the wave fronts slows 

 down as the wave fronts enter the denser 

 medium (water) but they remain parallel. 



DIRECTION 

 OF LIGHT RAY 



209.19 

 Figure 12-18. — Wave front diagram illustrating 

 the difference in the speed of light in air and 

 water. 



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