Effective ways of altering the speed of EMR

What are the ways in which the speed of Electromagnetic radiation can be effectively changed from point A to point B? (no relativistic motion) So far i have (i think):

!) Altering the direction
Light bouncing down a tunnel of mirrors

2) Altering the medium which it is travelling through

Does anyone know of any other ways? I am not sure of the effect of the space-time continuum on it; does it only change its direction temporarily? Can it be put back on the same course with multiple gravitational sources? Also will either of these change the wavelength?

Thanks! :)
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