Why does a 90° phase difference in light waves cause slower travel?

[einstein1921]

I read in a book!If the electrons emit a light wave which is 90° out of phase with the light wave shaking them, it will cause the total light wave to travel more slowly. BUT I don't know why?anybody can help? Thank you!

 

[pumila]

I assume that you are describing an energy storage mechanism which stores the photon energies and release them a quarter wavelength later.

The light wave will effectively be slowed down by the quarter-wavelength photon storage time.

Is this relevant to the question?