286 OUR PHYSICAL WORLD 



Some of the principles that underlie reflection are matters 

 of familiar experience. You know that when one looks at 

 himself in a mirror his right hand seems to be on the left side 

 of his image. If his hair is parted on the left, the image wears 

 its parted on the right. If he winks his right eye, the image 

 winks its left. A person and his mirror image face each other 

 in the same relative position as two persons facing each other. 

 A movement of the right hand toward the right appears in the 

 image as a movement of its left hand toward the left. We have 

 grown so accustomed to performing certain actions before the 

 mirror, such as combing the hair or tying a tie, that we are not 

 confused by the reversal. But undertake some unusual task, 

 looking at your action in the mirror, and it is difficult. Thus, as 

 you sit at the table, stand a book on edge on the table in front of 

 you. Behind it on the table lay a piece of writing paper. Stand 

 a mirror on the table beyond the paper. Now place your hand 

 on the paper ready to write and adjust the mirror so you can 

 see your hand and what you write, in the mirror, but cannot see 

 them by direct vision because the book is in the way. Then 

 write your name so you can read it in the mirror. 



It is a more or less familiar fact that the image as seen in 

 a plane mirror seems as far back of the mirror as the object 

 is in front of it. We all know, too, how curved mirrors distort 

 images. As a child you probably amused yourself by looking 

 at your face in the back of a shiny spoon and then in its bowl, 

 seeing your distorted image upright at first and then upside 

 down. All these phenomena pertaining to mirrors are easily 

 understood when one fixes in mind a very simple law, namely, 

 that the ray of light which strikes a reflecting surface is sent 

 off from it at the same angle at which it strikes, or, in other 

 words, the angle of reflection equals the angle of incidence. 

 This will be appreciated by a simple experiment. Stand a 

 mirror on a table so that the surface of the mirror is at right 

 angles to the surface of the table. On the table in front of the 

 mirror lay a sheet of paper, one edge against the edge of the 



