Why does frequency remain constant when EM waves change media?

[Swapnil]

I was wondering, when an EM wave goes from one medium to another why is it that the frequency remains constant?

 

[jtbell]

It's because of the boundary conditions that the electric and magnetic fields must satisfy.

http://farside.ph.utexas.edu/teaching/em/lectures/node70.html

http://farside.ph.utexas.edu/teaching/em/lectures/node76.html

 

[Claude Bile]

The perpendicular component of the electric displacement vector D, must be equal at the boundary AT ALL TIMES to preserve continuity - this is only possible if the frequency of the wave on both sides of the boundary is the same.

Claude.

 

[Swapnil]

Makes sense. Thanks!