At the point M, one or two inches in front of the mirror, 

 fix a pin vertically, and at the point N, two or three inches 

 to the right of M, fix a second pin. Images of these pins 

 can be seen reflected in the mirror or in the blackened glass 

 which serves as a mirror. Move the head until the image 

 of the pin at M seems to lie directly behind the pin at the 

 point N. Then fix another pin directly in front of N at 

 some point P, so that the image of the pin at M, the pin at 



N t and the pin at 

 B P all appear to be 



in the same straight 

 line. 



Remove the mir- 

 ror and pins and 

 join N and P by a 

 straight line, and 

 prolong this line un- 

 til it intersects AB 

 at O. Join M and O 



and erect a perpendicular OR at the point O. MO is 

 the incident ray, and the angle of incidence is the angle 

 between MO and the perpendicular OR, erected at the 

 point where the incident ray meets the mirror. PO is 

 the reflected ray, and the angle of reflection is the angle 

 between PO and the perpendicular erected as before. 



Measure off equal distances OS and OTon OMand OP, 

 respectively, and draw ST cutting OR at V. Measure 

 SFand TV very carefully. If they are equal, the angle 

 SO V equals the angle TO V, because by geometry if OS 

 = O T and SV ' FT^and OV is common-, the angle SO Fis 

 equal to the angle TO F. State the laws showing the relation 

 between the angle of incidence and the angle of reflection. 



FIG. 40. Reflection of light. 



