on the other hand, no transmission of light takes place. The angle of incidence corresponding to an angle of refraction 90º, say AO N, is called the critical angle ( i c ) for the given pair of media. We see from Snell’s law [Eq. ( .
)] that if the relative refractive index is less than one then, since the maximum value of sin r is unity, there is an upper limit to the value of sin i for which the law can be satisfied, that is, i = i c such that sin i c = n ( . ) For values of i larger than i c , Snell’s law of refraction cannot be satisfied, and hence no refraction is possible. The refractive index of denser medium with respect to rarer medium will be n = /sin i c . Some typical critical angles are listed in Table .
. FIGURE . Refraction and internal reflection of rays from a point A in the denser medium (water) incident at different angles at the interface with a rarer medium (air). A demonstration for total internal reflection All optical phenomena can be demonstrated very easily with the use of a laser torch or pointer, which is easily available nowadays.
Take a glass beaker with clear water in it. Stir the water a few times with a piece of soap, so that it becomes a little turbid. Take a laser pointer and shine its beam through the turbid water. You will find that the path of the beam inside the water shines brightly.
Shine the beam from below the beaker such that it strikes at the upper water surface at the other end. Do you find that it undergoes partial reflection (which is seen as a spot on the table below) and partial refraction [which comes out in the air and is seen as a spot on the roof; Fig. . (a)]?
Now direct the laser beam from one side of the beaker such that it strikes the upper surface of water more obliquely [Fig. . (b)]. Adjust the direction of