IMPORTANT THINGS ABOUT LIGHT 



105 



What happens when light strikes an object? Three 

 things may happen to light rays when they strike 

 an object. Some of the light may pass through the 

 substance, some will be absorbed, and some will be 

 reflected. Substances which allow enough light to 

 pass through them so that objects can be readily seen 

 are called transparent bodies. Glass, water, and air are 

 good examples. Substances which transmit light, but 

 not enough to distinguish objects through them, are 

 translucent bodies such as ground glass, oiled paper, 

 and cotton cloth. Substances which transmit no light 

 are called opaque bodies. All substances reflect some 

 of the light which strikes them and all absorb a part 

 of it. A mirror is an object that reflects nearly all the 

 light it receives. 



Nearly every person has experienced that helpless 

 feeling at night in a dark room when one is unable 

 to see anything. We are able to see an object only 

 when light is present and reflected by the object to 



FIG. 157. DIFFUSE AND REGULAR REFLECTION 



our eyes. Most objects have rough surfaces so that 

 the rays of light which strike them are reflected in an 

 irregular manner, being thrown off in different direc- 

 tions as shown in Figure 157. This is called diffuse 

 reflection and produces diffused light. When parallel 

 rays of light strike a very smooth, flat surface, how- 

 ever, the light is reflected to our eyes as parallel rays 

 as shown in the figure. This is called regular reflection 

 and makes it appear as if the light were coming from 

 a distant object rather than from a reflecting surface. 

 In other words, we see the image of the source of light 

 rather than the reflecting object. Any object so highly 

 polished that it will produce an image of the source 

 of light is called a mirror. 



Diffuse reflection plays an important part in our 

 everyday life. Sunlight is reflected in all directions by 

 smoke and dust particles in the atmosphere. If this 

 condition did not exist we should see only the bril- 

 liance of the sun and darkness everywhere else. Thus 

 we see that light is reflected several times. It is re- 

 flected first by the "sky," then by objects on the 

 earth, and indoors by the walls of the rooms of our 

 houses. In this way a soft, diffused light is produced 

 which is more pleasing to the eye than the glaring 

 light direct from the sun. 



What is the law of reflection? Mirrors are some- 

 times placed at dangerous road turns to enable cars 

 coming from the other direction to be seen. Have 

 you ever observed that rear-view mirrors used in auto- 

 mobiles are set at an angle so that light coming 

 through the back window may be reflected to the eye 

 of the driver. A ray of light which strikes a mirror 

 is sometimes called an incident ray and the ray which 

 leaves the mirror, the reflected ray. If a line is drawn 

 at right angles to a mirror where the incident ray 

 strikes, as shown in Figure 158, the angle between 



Incident ray 



Reflected ray 



FIG. 158. ANGLES OF INCIDENCE 

 AND REFLECTION 



the perpendicular line and the incident ray will be 

 found exactly equal to the angle between the reflected 

 ray and the perpendicular line. This is known as the 

 law of reflection. 



Under what conditions are rays of light refracted? 

 Have you ever noticed that the bottom of a dish of 

 water or the bottom of a stream appears to be nearer 

 the surface than it really is? A straight stick held 

 slantingly in the water and looked at from the side 

 seems to be bent upwards at the point where it enters 

 the water. It was stated previously that light travels 

 in straight lines, but this is true only when light is 

 passing through a medium of a fixed density. When 

 light passes at an angle from a medium of one density 

 to a medium of a greater or lesser density, the light 

 rays bend. This ac- 

 counts for the apparent 

 shifts of the positions 

 of objects when seen 

 under water. This bend- 

 ing of light rays is 

 called refraction. The 

 general rule is that 

 light passing at an an- 

 gle from a dense to a 

 less dense medium, as 

 from water to air, is 

 bent away from the per- 

 pendicular, while light passing at an angle from a 

 medium of lesser density to one of greater density 

 is bent toward the perpendicular. See Figure 160. 



Exercise. // you wished to shoot fish in a clear stream, 

 ivould you aim a gun above or below it to make certain 

 of hitting it? Why? 



What is the composition of sunlight? We have 



FIG. 159. REFRACTION OF LIGHT 

 BY WATER 



