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2014-12-02T17:24:18+05:30
the beam of light deviates  while moving from its path when it enters a glass slab due to refraction, which is caused due to the difference in the velocity of light in different medium due difference in densities.

the light does not bend if it enters at right angles because according to the Snell's law: μ=sin i/sin r
       when angle of incidence is 0⇒sin 0=0
⇒sin r=0/μ
⇒sin r=0⇒ angle of refraction is ⇒0°.
So there is no shift in the path of light and the light passes through undeviated
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2014-12-04T22:19:35+05:30
The light when enters the glass at an angle it bends due the phenomenon called refraction. Due to change of medium the phase velocity of light  changes . this is governed by law which states sinФi/sinФr=vi/vr.
when light is perpendicular sinФi is zero
Since the incident angle is 0 the angle of refraction is also zero.
Though the speed of light reduces in glass medium this can not be noticed.
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