Answer:
[tex]3.031\times 10^{-5}\ \text{m}[/tex]
Explanation:
[tex]y[/tex] = Distance between central maxima and first minimum
m = Order = 1
d = Thickness of hair
[tex]\lambda[/tex] = Wavelength = 632.8 nm
L = Distance between light source and screen = 1.25 m
Width of central maximum is given by
[tex]2y=5.22\times 10^{-2}\\\Rightarrow y=\dfrac{5.22\times 10^{-2}}{2}\\\Rightarrow y=0.0261\ \text{m}[/tex]
Distance between central maxima and first minimum is given by
[tex]y=L\tan\theta_{min}\\\Rightarrow \tan\theta_{min}=\dfrac{y}{L}\\\Rightarrow \tan\theta_{min}=\dfrac{0.0261}{1.25}\\\Rightarrow \theta_{min}=\tan^{-1}0.02088\\\Rightarrow \theta_{min}=1.1962^{\circ}[/tex]
Since [tex]\theta[/tex] is small [tex]\tan\theta_{min}=\sin\theta_{min}[/tex]
[tex]\sin\theta_{min}=\dfrac{m\lambda}{d}\\\Rightarrow d=\dfrac{m\lambda}{\sin\theta}\\\Rightarrow d=\dfrac{1\times 632.8\times 10^{-9}}{\sin1.1962^{\circ}}\\\Rightarrow d=3.031\times 10^{-5}\ \text{m}[/tex]
The strand of hair is [tex]3.031\times 10^{-5}\ \text{m}[/tex] thick.
