LESSONS IN PHYSICS. 75 



by reflection to a focus at the other, they are nailed conjugate 

 foci. Parallel rays, as I A and KB falling on the mirror at 

 A and B are reflected to F, a point midway between C and G, 

 which is called the principal focus; while rays diverging from 

 the principal focus would be reflected as parallel rays, and the 

 converging rays LA and PB are reflected to S. This shows that 

 the general effect of the concave mirror is to converge the rays of 

 light they reflect, and they converge rays of heat as well, so that 

 a piece of paper or a chip of wood at the principal focus, may be 

 burned in the collected rays of the sun. 



The reflector of a locomotive headlight is a parabolic mirror. 

 The light is placed at the principal focus, so that the reflected 

 rays are... parallel. 



The general effect of convex mirrors is to separate rays of 

 light. 



The image of an object made by a plane mirror appears to be 

 behind the mirror as far as the object is in front of it. It 

 appears to be of the same size as the object and is erect that is, 

 the plane mirror gives a perfect image of the object. 



In the case of the concave mirror, if the object is beyond the 

 center of curvature the image will be between the center of curva- 

 ture and the principal focus, and will be inverted and smaller than 

 the object. If the object is between the center of curvature and 

 the principal focus, the image will be beyond the center of curva- 

 ture, inverted and larger than the object. The image of an object 

 between the principal focus and the mirror will be back of the 

 mirror, erect and larger than the object. 



The images formed by the convex mirror are always erect, 

 behind the mirror and smaller than the object, and are generally 

 curiously distorted. 



Many valuable and interesting experiments may be made with 

 cheap plane and curved mirrors, which will serve to verify these 

 laws. A bright silver spoon may serve both as a concave and a 

 convex mirror ; so of metal cups, bells and other articles of com- 

 mon use. 



Light moves with different velocity, through different trans- 



