164 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



from a plane mirror, that is, a flat reflecting surface. Such a 

 mirror can be made from a variety of substances, but the most 

 common is bright metal or silvered glass. To find the relation 

 between the angle at which a beam strikes a mirror and the 

 angle at which it is reflected, the following experiment should 

 be performed. 



EXPT. 162. Fix two slabs of wood at right angles as in 

 Fig. 73, AB, CD. Against the upright slab place a piece of 



Fio. 73. Arrangement to illustrate the Law of Reflection. 



glass EF with blackened back so that reflection only takes 

 place from the front. Upon the horizontal slab place a sheet of 

 white paper. Stick a pin b in the wood against the glass, and 

 place another pin near the position a. Now procure another 

 pin and stick it into the wood at c in such a position that c, 6, 



and the image of a are in 



a straight line. Draw 

 with a finely pointed 

 pencil a line along the 

 edge of the glass xy ; 

 then take glass and pins 

 away. 



The paper will be 

 marked by the pin-holes 

 and the line xy. Draw 

 FJG. 74. Angles of Incidence and Reflection. lines through the pin- 



holes, and at b a normal 



to xy, that is, a line perpendicular to xy. Measure the angles 

 abd, cbd, and compare them (Fig. 74). Repeat the experiment 

 two or three times, with the pins in different positions, and so 

 determine that the angle of incidence and the angle of reflec- 

 tion are equal. 



