LIGHT RAYS AT A BOUNDARY

A qualitative example of refraction is shown in Fig. 1, where the refractive index of the first (lower) medium is higher than that of the second (upper) medium. A ray striking the boundary at a right angle (that is, angle of incidence equal to 0°) passes through without changing direction. However, a ray that hits at some other angle is bent; the greater the angle of incidence, the sharper is the turn the beam takes. When the angle of incidence reaches a certain critical angle, then the light ray is not refracted at the boundary but instead is reflected back into the first medium. This is known as total internal reflection.
A ray originating in the second (upper) medium and striking the boundary at a grazing angle is bent downward. This causes distortion of landscape images when viewed from underwater. You have seen this effect if you are a scuba diver. The sky, trees, hills, buildings, people, and everything else can be seen within a circle of light that distorts the scene like a wide-angle lens.
If the refracting boundary is not flat, the principle shown by Fig. 1 still applies for each ray of light striking the boundary at any specific point. The refraction is considered with respect to a flat plane passing through the point and tangent to the boundary at that point. When many parallel rays of light strike a curved or irregular refractive boundary at many different points, each ray obeys the same principle individually.

Fig. 1. Rays of light are bent more or less as they cross a boundary between media having different properties.