No Bending Along Normals
Why does a light ray not bend when travelling through two
media having different relative refractive indices when the ray travels
perpendicular to their surfaces?
The bending of light in going from medium to another has to
do with the different speeds of light in different materials.
When hitting an interface at an angle, the entire wave does not
hit the interface simultaneously or symmetrically. (Which it is,
depends upon whether you think of plane wave fronts or wave fronts
with obvious curvature.
One part of the wave enters first and the wave front must bend to
account for this. If a wave in normally incident, any effects
are either simultaneous or symmetrical. There is no bending.
Think of a vehicle pulling off a road into a muddy verge. As soon
as the right wheels hit the mud, the car tries to turn right because
those two wheels are going slower than the ones on still on the pavement.
If you drive from the pavement into the mud at a right angle, both
front tires hit at the same time. You slow down but there are no
turning forces involved.
Hope that helps.
R. W. "Bob" Avakian
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology
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Update: June 2012