Consider a plane wave front travelling in vacuum incident on the surface of a block of glass at an angle of incidence i as shown in figure 9. The line XW is the section of the wavefront on the plane of the paper. Note that XW is only a portion of the wavefront (WF) moving along the incident ray SO. Due to the inclined incidence the point X reaches the surface first. Let us take this instance as t=0. As the WF enters glass, the portion of the WF in vacuum travels with speed c and the portion in glass travels with speed v which is less than c. Therefore the WF at successive instances are closer to each other in glass than in vacuum. When the point W touches the surface of glass at y, the refracted part of the section XY is represented by VY. The incident ray is along SO (perpendicular to XY), the refracted ray is along OT (perpendicular to VY). The angles of incidence and refraction are related to the speed of light as shown in the box below.
3 B. Why light bends due to change in speed
Snell's laws of refraction are the result of a careful experimental study of the behavior of the incident, reflected and the refracted rays. The value of the ratio derived by Eq. 3 is same as the ratio of the speed of electromagnetic wave in vacuum to its speed in the given medium; this is the modern accepted definition of μ (Eq. 1, section 2). It may be puzzling to note that two ratios, one derived from the angles and another from speeds are indeed equivalent. For a curious student, the following section gives a geometrical proof. The ray of light is in fact the direction of propagation of the light energy. Light is electromagnetic in nature, the variations in the electric and magnetic vectors are perpendicular to each other and are confined to a plane which is perpendicular to the ray. This plane is referred to as the wavefront. A two dimensional section of a spherical wavefront from a point source is shown in Figure 7. Energy along parallel rays is carried along a plane wavefront as shown in Figure 8. The wavefront moves with the velocity of light.
