406 THE OUTGO OF ENERGY 



rays are converged, is the principal focus of the 

 concave mirror. The distance between the prin- 

 cipal focus and the reflecting surface is termed 

 the principal focal distance; it is one half the 

 radius of curvature. Accurate measurement 

 would show that the angle between the incident 

 ray and the perpendicular, 1 in this case the radius 

 of the spherical surface, equals the angle of 

 reflection. 



2. Take tHe mirror from the box and set it 

 in the principal axis of the beam coming from 

 the lantern. Eeplace the 2 mm. diaphragm by 

 the diaphragm with |_-shaped aperture. At the 

 principal focus of the concave mirror hold the 

 small round screen with slender handle. 



The inner rays of the beam will be inter- 

 cepted by the screen. The outer rays will be 

 reflected from the mirror and an inverted, real 

 image of the |_-shaped aperture will be seen upon 

 the screen. The image will be smaller than the 

 object. When the distance between the mirrpr 

 and the object is less than the radius of curvature 

 but greater than the focal distance, the image 

 is real, inverted, and larger. With concave 

 mirrors, real images are always inverted. 



and the reflected beam may be compared by turning the mirror 

 slightly, so that they lie side by side. 



1 It is assumed that the spherical surface is composed of an 

 infinite number of plane surfaces. 



