Angle of Incidence: Subtracting 90 Degrees Explained

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Discussion Overview

The discussion revolves around the concept of the angle of incidence, specifically addressing why it is common practice to subtract from 90 degrees when calculating this angle. Participants explore different methods and contextual understandings, including potential variations in practice in different regions, such as Australia.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the necessity of subtracting from 90 degrees to determine the angle of incidence.
  • Another participant questions the assumption that the method of subtracting from 90 degrees is universally applied, suggesting that there may be alternative methods used in Australia.
  • A participant clarifies that the angle of incidence is defined as the angle from the normal to the ray, but questions the relevance of the Australian context.
  • One participant argues that the question of why subtraction is necessary may be poorly posed without specifying the method being used, indicating that there could be other valid approaches to determining the angle of incidence.
  • It is noted that the inquiry may also relate to a broader question about the customary focus on the angle of incidence versus the grazing angle.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the necessity of subtracting from 90 degrees, and multiple competing views regarding the methods of calculating the angle of incidence remain unresolved.

Contextual Notes

Participants do not provide specific details about the methods they are using to find incidence angles, which may limit the clarity of the discussion.

yakin
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I do not understand why we do we subtract from 90 degrees when finding the final answer for angle of incidence?
 
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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?
 
Evgeny.Makarov said:
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?
 
yakin said:
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.
 

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