6 PROPERTIES OF LENSES. 



All rays falling on Convex lenses are refracted 

 towards the centres of curvature of the faces, those 

 incident on Concave away from the centres, (Figs. 3 

 and 7). 



In convex lenses the PRINCIPAL FOCUS (F. Fig. 3) is 

 that point where all rays parallel before entrance 

 converge after refraction, it almost coincides with 

 the centres of curvature of the faces. 



If the rays diverge from a point A outside the 

 principal focus, they meet after refraction on the 

 other side beyond it B, these points A and B are 

 real foci (Fig. 3). 



If they converge they meet at C within it, B and 

 C are called conjugate foci. 



Eays diverging from a point C Fig. 3, within the 

 principal focus of a convex lens, can never meet at 

 the opposite side, but only become less divergent, 

 they therefore cannot form a real focus. But if the 

 emergent rays are prolonged backwards they will 



FIG. 3. 



meet at V Fig. 3. The point V is called the virtual 

 focus. 



