For a grating having 350 rulings/mm, longest wavelength is mathematically given as
lambda_max=3.704*10^{-7}m
Generally, the equation for the wavelength is mathematically given as
lambda=dsin\theta/6
Therefore, for lamda_max theta=\pi/2
lambda_max=dsin\pi/2 /6
In conclusion
lambda_max=d/6
lambda_max=3.704*10^{-7}m
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