A) Given,
n = 3
λ=650 nm
D=1.2 m
d=2 mm
Distance of nth bright fringe from central maxima:
Y=nλDd
Y=3×1.2×103×650×10−62
Y=1.17mm
Final Answer : 1.17mm.
B) Given,
λ=650 nm
D=1.2 m
d=2mm
Distance of nth bright fringe from central maxima:
Y=nλDd
Y∝λ
At a distance Y, nth bright fringe of 520 nm coincides with (n−1)th bright fringe of 650 nm,
Ynth=Yn−1th
nλ1Dd=(n−1)λ2Dd
nλ1=(n−1)λ2
Putting values
520 n=(n−1)650
n=5
Distance Y from central maxima,
Y=5×520×10−6×1.2×1032
Y=1.56 mm
Final Answer: 1.56 mm.