## Young's double slit angles

I am trying to prove that the two θ are equal from this picture

http://en.wikipedia.org/wiki/File:Double_slit.svg

How do I go about proving this?
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 This might help: The small triangle's theta refers to the top angle of the triangle, not the left one. Also, the right angle is right. By that I mean that the angle on the right side of the triangle is a 90 degree angle. This post was an excuse to say that the right angle is right.
 Yes I was aware of which angles where which. I just used that picture since it was easily attainable. Any hints as to starting this

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## Young's double slit angles

Suppose you have two lines A and B that have angle θ between them.

If you have line C parallel to A, and line D parallel to B, then C and D also have angle θ between them, right?

Likewise, if you have line E perpendicular to A, and line F perpendicular to B, then E and F also have angle θ between them.
 Recognitions: Gold Member Science Advisor The phrase 'similar triangles' comes to mind and it hasn't yet been spelled out.