I believe it is the first, because the rotation is reversed by the medium, but i

Question

I believe it is the first, because the rotation is reversed by the medium, but is that actually correct?

Assume polarised light travels through an optically active medium (e.g. water with lots of sugar) with the specific angle alpha of depth d at which end it is reflected (so in total it travels 2d). Will the light at the end where it initially entered the material be polarised with a relative angle of alphasd +(-alphas)d=0 or with alphasd+alphasd=2alphasd I believe it is the first, because the rotation is reversed by the medium, but is that actually correct?

Explanation / Answer

Correct, the net polarization rotation is zero. There's a neat way to see this. When you calculate the effects of a mirror, you can always do the calculation by imagining that the mirror is not a mirror, but a portal into an (imaginary) mirror-image copy of everything on the other side of the mirror.

An atom-by-atom mirror image of a solution of D-glucose is a solution of L-glucose.

Therefore, you imagine shining the light through a distance d of D-glucose, then through the mirror into the imaginary mirror-world, then through a distance d of L-glucose, then out the other end.

Obviously the rotation of D-glucose cancels the rotation of L-glucose, so the net rotation is zero.

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