416 THE OUTGO OF ENERGY 



REFRACTION BY CONVEX LENSES 



Principal Focus. 1. Place the diaphragm of 

 2 in in. aperture in the lantern. Throw a beam 

 of parallel rays into the optical box. Place the 

 double convex lens in the axis of the beam about 

 5 cm. from the window of the box. 



The parallel rays will be brought to their 

 principal focus about 10 cm. (4 inches) from the 

 lens. Note the increase in intensity as the rays 

 converge. 



Place the black wooden screen at this point. 



A real image of the luminous aperture of the 

 diaphragm will be perceived. 



2. Place the diaphragm of 2 mm. aperture over 

 the window of the box. Direct the light of the 

 lantern upon the opening in the diaphragm. 

 From the illuminated opening rays will diverge 

 in all directions. Place the lens 10 cm. from this 

 luminous body, so that it shall lie in the princi- 

 pal focus of the lens. 



The diverging rays will be rendered parallel. 

 Rays diverging from the principal focus are 

 rendered parallel by passing through a convex 

 lens. 



A. lens may be regarded as an infinite series of 

 prisms. In a convex lens the refracting angle 



