Can anyone explain the answer to this question please?

[whatwhy]

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The beam splitter in figure 1 is set at what angle to the incident beam? It's 45 degrees.

 

[sazerac]

If you go back to your physics book, it will say that for reflected light, theangle of incidence equals the angle of reflection. The passage says that the reflected light reflects "at a right angle", which is 90 degrees. Thus the angle of incidence (and the angle of reflection) are each 45 degrees, which sum up to the 90. The question asks what the angle of incidence is.

https://en.m.wikipedia.org/wiki/Reflection_(physics)