We have already learned that a lens is a carefully ground or molded piece of transparent material which refracts light rays in such as way as to form an image. Lenses serve to refract light at each boundary. The net effect of the refraction of light at these two boundaries is that the light ray has changed directions. Because of the special geometric shape of a lens, the light rays are refracted such that they form images. Before we approach the topic of image formation, we will investigate the refractive ability of converging and diverging lenses.
First lets consider a double convex lens .
Upon reaching the front face of the lens, each ray of light will refract towards the normal to the surface. This is shown for two incident rays on the diagram below. Once the light ray refracts across the boundary and enters the lens, it travels in a straight line until it reaches the back face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. The above diagram shows the behavior of two incident rays approaching parallel to the principal axis.