Given, slit separation, d=0.3mm=0.3×10−3m Distance between slit and screen, D = 1.5 m Distance between 4th bright fringes =2Y4=2.4cm Position of 4th fringe, Y4=
2.4
2
=1.2cm=1.2×10−2m Using equation, Yn=
nλD
d
For n = 4 1.2×10−2=
4×λ×1.5
0.3×10−3
∴ Wavelength, λ=6×10−7m Using c = λf ⇒f=
c
λ
=
3×108
6×10−7
=5×1014Hz Hence, the frequency of light is 5×1014Hz. So, correct answer is 5.