For the central bright fringe, the path difference is 0.
- S1O = 0.
For the first bright fringe
- S1Q =
Notice that in the triangle S1S2Z, S2Z is l.
We know that S1S2 is s.
For the triangle OPQ,
Although in this diagram, it is clear that
f, in the real thing, we can assume that
as the real set up is very much longer.
We know that for small angles
q = tan
To produce easily measurable
fringes, D must be large i.e. in
metres while s is small (<1 mm).
The larger D is the less bright the
fringes. We can increase the width
of the fringes by increasing the wavelength, or by decreasing the slit width.
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