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Question:
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?


Replies:
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
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology



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