LIGHT 



3424 



LIGHT 



Fig.2 



they come. In this connection it is customary 

 to use the term ray to designate the path along 

 which the waves travel. We can see bodies 

 with a rough surface readily because the light 

 rays are reflected irregularly, or diffused (see 

 Fig. 1). That is, the reflected rays are scattered 

 in every direction, and the body can be seen 

 from any point. 



When the surface of an object is very smooth, 

 like that of a mirror, little if any light is dif- 

 fused. Instead, most of the rays are reflected 

 regularly and uniformly in a definite direction, 

 and the light seems to come to the eye from 

 distant objects instead of from the reflecting 

 surface. That is why one sees his own image 

 in a mirror, while the latter is itself invisible. 

 In every case the image appears to be as far 

 behind the mirror 

 as the object is in 

 front of it. In 

 Fig. 2, let A B 

 be an arrow held 

 in front of the | 

 mirror m n. Rays 

 of light from the 

 point A strike A 

 upon the mirror 

 at D, are reflected and enter the eye as if 

 they came from A'. Rays from B seem to 

 come from B'. The image, because it is seen 

 in the direction of the reflected rays, appears 

 at A'B', which is as far behind mn as the 

 arrow is in front of it. Such an image is 

 known as virtual, because it has no actual 

 existence apart from the eye of the observer. 



The angle at which a ray strikes a reflect- 

 ing surface is called the angle of incidence, and 

 that at which it is sent back is called the angle 

 of reflection. Comparing these angles, scien- 

 tists have discov- 

 ered the law cf 

 reflection, as fol- 

 lows : The angle 

 of incidence is 

 equal to the 

 angle of reflec- 

 tion (see Fig. 3). 

 It is interesting 

 to note that the 

 reflected ray has 

 the same relative 



position as the striking ray, just as a ball 

 thrown against a wall at an acute angle bounds 

 away from the wall at the same angle. 



Refraction of Light. We know that light 

 travels in a straight line so long as it is pass- 



Ffg.3 



ing through any one medium. Let us see what 

 happens when a ray strikes a medium of a 

 different density than the one in which it 

 started. The straw through which you drink 

 your lemonade looks bent at the surface of the 

 liquid, and so does the spoon in a cup of clear 

 tea. This is because light is bent or refracted 

 from its straight course when passing from 

 one medium to another of different density. 

 There are two important laws to remember in 

 connection with refraction: 



(1) When light passes from a rare to a dense 

 medium, it is bent in the direction of a line per- 

 pendicular to the surface of the refracting body. 



(2) When it passes from a dense to a rare me- 

 dium it is bent away from such a line. 



These laws are illustrated in Fig. 4, which 

 shows the path of rays through a windowpane. 

 When a ray enters the glass it is refracted 

 toward the per- 

 pendicular, and 

 on leaving, is 

 bent from the 

 perpendicular; if 

 the glass is clear 

 the refraction is 

 equal in both di- 

 rections, and the 



objects seen >y pjg 4 



through it appear 

 natural. An 

 even glass, 

 causing unequal refraction, would make the 

 objects appear distorted. 



In nature we see many examples of the prin- 

 ciples of refraction. A mirage in the desert is 

 often caused by the bending of the light rays 

 as they pass from denser to rarer layers of 

 air. The setting sun is viewed after the glow- 

 ing orb has passed below the horizon, for the 

 rays from it which reach the eye are bent 

 downward, as they pass through the atmos- 

 phere of the earth, .while the eye follows them 

 back in a straight line. For the same reason 

 we see the sun in the early dawn before it is 

 really above the horizon. Rainbows are caused 

 by refraction and reflection. Another illustra- 

 tion of refraction is the wavy appearance of 

 hot air rising above a stove; the rays of light 

 are bent as they pass through layers of air of 

 varying densities. Again, if you look at a 

 clear stretch of water from the side the bot- 

 tom will seem to be raised, giving the effect of 

 shall owness. This is due to the bending of 

 the light rays as they pass from beneath the 

 water into the air. 



