Good day to all, I the following diagram is a non-polarized beam splitter cube with a 10:90 (R:T) split ratio. The questions I have are:

  1. that when light comes back does the 90°, R path still have a 10% portion and the T path have 90%?

  2. Would a plate beam splitter of the same ratios behave the same way?

Beam Split Ratio Diagram

Many thanks in advance...


The following assumes that the beamsplitter cube or plate is made of isotropic material like glass, and that the refractive index of both halves of the cube is the same. Birefringence and polarization can complicate the story.

So: imagine removing the arrows that are inside the beamsplitter (they just confuse the picture) but leave the arrows on the transmitted and reflected beams.

Now imagine flipping the diagram on the left first on a vertical axis, then on a horizontal axis.

That flipping process does not change anything about the physics of the situation, but it makes the flipped diagram identical to the diagram on the right. Now you can easily see the % of light transmitted and reflected.

A plate beamsplitter (one face antireflection coated, the other face thinly aluminized) will work essentially the same way: the transmitted-to-reflected beam ratio will be the same regardless of whether the beamsplitter is used in the forward or backward mode.

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  • $\begingroup$ The flips that you suggest do not bring the BS to a configuration that is identical to the second figure. The order of the layers in the BS coating will be different, and if there is cement, if the light hits the BS coating in the drawing on the left, but in the drawing on the right the light hits the cement first. It may be true that the splitting is identical in the two cases, but your argument does not lead to that conclusion. $\endgroup$ – garyp Sep 5 '19 at 2:32
  • $\begingroup$ Of course, the best way to get an answer is to do the experiment. $\endgroup$ – S. McGrew Jan 9 at 13:57

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