Question

A parallel beam of light from a He-Ne laser, with a wavelength 633 nm, falls on two very narrow slits 0.070 mm apart.

Part A

How far apart are the fringes in the center of the pattern on a screen 4.1 m away?

Answer 1

for constructive interference in two slit experiment we know that

m* = d*sin

for small angles we know that

sin = = tan = y/D

So,

m* = d*y/D

y = m**D/d

Given values are:

D = distance between screen-slit = 4.1 m

d = distance between two slits = 0.070 mm

wavelength = 633 nm

Now distance between two consecutive fringes will be:

dy = dm**D/d

for two consecutive fringes, dm = 1 (since (m + 1) - m = 1)

So

dy = 1*633*10^-9*4.1/(0.070*10^-3)

dy = 0.0371 m = 37.1*10^-3 m

dy = distance between fringes = 37.1 mm

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