Why Is the Fringe at the Narrow End of a Wedge Always Dark?

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SUMMARY

The discussion focuses on the phenomenon of dark fringes at the narrow end of a wedge in optical interference patterns. It emphasizes the importance of understanding phase change upon reflection and the distance light travels when passing through the wedge. The equation for the distance between dark fringes, denoted as /\x, is derived from these two critical factors. The conclusion highlights that the interference effects are fundamentally linked to the geometry of the wedge and the properties of light reflection.

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For a solid wedge of material of length L and maximum width D (ie, a triangle) how would one obtain an equation for /\x, the distance between dark fringes?

For wedge fringes, how come the fringe at the narrow end is always dark?
 
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You need to consider two factors that determine the phase difference--and thus the interference--between the reflections at each boundary:

What is the phase change upon reflection?

What distance does the light travel in passing (twice) through the wedge?​
Read up with these factors in mind.
 

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