Answer:
L = 0.93 meter
Explanation:
As we know that the diffraction pattern will show the minimum on the screen at the position where it follows the equation
[tex]a sin\theta = N\lambda[/tex]
here we know that for second fringe position we have
[tex]sin\theta = \frac{2\lambda}{a}[/tex]
[tex]sin\theta = \frac{2(687 nm)}{0.75 mm}[/tex]
[tex]sin\theta = 1.832 \times 10^{-3}[/tex]
[tex]\theta = 0.105 degree[/tex]
now we know that the position of minimum is given at 1.7 mm from the center
so we have
[tex]tan\theta = \frac{y}{L}[/tex]
[tex]tan(0.105) = \frac{1.7\times 10^{-3}}{L}[/tex]
[tex]L = 0.93 meter[/tex]