Light always follows a straight line during its propagation. I think everything, either a particle or wave having equal or higher speed than light must follow a straight line. Is there any controversy on this topic?
As I understand, the reduced effective speed of light in a material medium actually represents delay introduced by the absorption and any subsequent reemission of photons by the material's individual atoms. The speed of light between atoms (when reemitted) is c. See http://en.wikipedia.org/wiki/Speed_of_light#In_a_medium.
I think it's generally considered in relativity that gravitational lenses are the result of light linearly propagating through curved spacetime...
I think it should be considered that all particle/waves always propagate linearly - that non-linear motions are the result of inertia-limited propagation durations and subsequent independently directed propagation periods - but that only perpetually propagating zero rest mass particles continuously propagate linearly. Please see http://en.wikipedia.org/wiki/Matter_wave.
Light doesn't follow a straight line except when in the absence of gravity and any other nonlinear media. Consider for instance the effect of gravity. It is generally known that gravity has the effect of bending light. This is because that gravity changes spacetime, and there is generally no straight line in spacetime with gravity. The less well-known aspect of gravity on light is that light does not even travel along light-like geodesics unless geometric optics approximation is adopted. See Robert Wald's General relativity book for details.
Light travels in straight lines ... unless it has a reason not to. Provide it with a reason to bend, and it'll bend. Same thing goes for the paths of freefalling objects - things generally don't change their state of motion spontaneously, without a cause.
The reason why we attach great significance to the idea that "light travels in straight lines" (as opposed to, say, "footballs travel in straight lines") is that the far higher speed of light (as opposed to the speed of a football) means that whereas the effect of gravity or a crosswind on the path of a kicked football is obvious, the corresponding angular change on a beam of light is so tiny that for everyday purposes, the deflection might as well not exist. However, when we carry out sensitive experiments, then yes, the existence of the Earth's gravitational field or air moving across the beam does cause a deflection.
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