Answer:
[tex]\lambda =533.6 nm [/tex]
Explanation:
the slits spacing, d = 0.21 mm
distance of screen, D = 61 cm
The condition for minima is given as
[tex]dsin(\theta) = \left ( n+\frac{1}{2} \right )\lambda[/tex]
So, first minima, n = 0
[tex]dsin(\theta_1) = \frac{1}{2}\lambda[/tex]
fifth minima, n = 4
[tex]dsin(\theta_5) = \frac{9}{2}\lambda[/tex]
[tex]d(sin(\theta_5) -sin(\theta_1))= 4\lambda[/tex]
For small angle
[tex]d(tan(\theta_5) -tan(\theta_1))= 4\lambda[/tex]
From the figure:
[tex]d(\frac{y_5}{D}-\frac{y_1}{D})= 4\lambda[/tex]
[tex]\frac{d}{D} (y_5-y_1) = 4\lambda[/tex]
[tex]\lambda = \frac{d}{4D} (y_5-y_1)[/tex]
[tex]\lambda = \frac{0.021}{4(61)} (6.2 \times 10^{-3})[/tex]
[tex]\lambda =533.6 nm[/tex]
