In the case of plane surfaces, a ray will bend towards the normal if it travels from a rarer to denser medium and it bends away from the normal if it travels from a denser to a rarer medium .
Let us see how we can draw ray diagrams for certain useful cases.
• What happens to a ray that travels along the principal axis?
Similarly, a ray that travels through the centre of curvature?According to Snell’s law the ray which travels along the normal drawn to the surface does not deviate from its path.
Hence both rays mentioned above travel along the normal (see figure 3),
so they do not deviate.
• What happens to a ray travelling parallel to the principal axis?
Observe the following figures 4a, 4b, 4c, and 4d.
In all the cases as represented by the diagrams, the incident ray is parallel to the principal axis.
Case1: A ray travelling parallel to the principal axis strikes a convex surface and passes from a rarer medium to denser medium. (see figure 4a)
Case2: A ray travelling parallel to the principal axis strikes a convex surface and passes from a denser medium to rarer medium.
(see figure 4b) axis strikes a concave surface and passes from a denser medium to rarer medium. (see figure 4c)Case4: A ray travelling parallel to the principal axis strikes a concave surface and passes from a rarer medium to denser medium.
(see figure 4d)• What difference do you notice in the refracted