Help with understanding Young's Derivation

  • Thread starter aeromat
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Homework Statement


Here they are deriving the equation for Young's experiment: http://www.physicsclassroom.com/Class/light/U12L3c.cfm

The part where they start bringing up "Assertion" and "Logic/Rationale" is where I got confused:
They mention for part ii) that if the distance L >>> y, then S1P is || (parallel) to BP.
u12l3c3.gif

iii

S1P = BP

If S1P and BP are || and line S1B is perpendicular to BP, then the length BP = length S1P.
If they are parallel to each other, wouldn't the two lines NOT connect together to form a triangle? What I know about parallel lines, they never intersect, and in this case, they are intersecting at P. :confused:

Homework Equations



Young's Equation for a double slit experiment
 

Answers and Replies

  • #2
cepheid
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They're not actually parallel, they're just close enough to parallel that you can pretend that they are parallel (without significant loss of accuracy). It's an approximation. The larger you make L in comparison to y, the "thinner" the triangle gets, and the better this approximation is.
 

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