Angle of Incidence: Subtracting 90 Degrees Explained

[yakin]

I do not understand why we do we subtract from 90 degrees when finding the final answer for angle of incidence?

 

[Evgeny.Makarov]

How do you know that in Australia they don't find the angle of incidence directly, without subtracting from 90°? Also, what is the the angle of incidence?

 

[yakin]

How do you know that in Australia they don't find the angle of incidence directly, without subtracting from 90°? Also, what is the the angle of incidence?

Angle of incidence is the angle from the normal to the ray. Whats up with Australia though?

 

[Evgeny.Makarov]

Whats up with Australia though?

You are leaving out essential context to your question. Asking why you need to subtract from 90° to find the angle of incidence is like asking why you need to do multiplication to compute $2\sin(\alpha)\cos(\alpha)$. The answer is that you don't have to do multiplication: you can use the formula $2\sin(\alpha)\cos(\alpha)=\sin(\alpha+\alpha)$. "Why do we have to use..." is generally not a well-posed question; it only makes sense when you restrict yourself to a certain method of solving your problem. There may be other methods (perhaps used in Australia) where one does not need the thing you are using. And since you did not describe the method you are using to find incidence angles or the type of problems, I am saying that you are leaving out important context.

Of course, you may be asking why it is customary to be interested in the angle of incidence instead of the grazing angle. That is a different question.