(i) Let’s assume the given postulate is true, then if A and B be two parallel rays entering to such a medium from air the refracted diagram will be FIG (1)
Here incidence angle is θi and refracted angle is θr . AB shows the incident wave front and ED shows the refracted wave front, then all the points on ED will have the same phase.
We know all the points with the same optical path length must have the same phase.
CD should be greater than 0, which is true according postulate diagram, which means rays of light enter such a medium from air (refractive index = 1) at an angle in 2nd quadrant, then the refracted beam is in the 3rd quadrant.
Let us consider the normal case where light entering 2nd quadrant refracting in 4rth quadrant as in FIG (2), then
which means if AE>CD, BC <0 which is not possible
the postulate is true
(ii) From FIG (1)
--Hence Snell’s law is proved
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