Answer:
Number of lines per mm grating =229
Explanation:
First of all, let's make use of the grating equation which is;
dsinθ=nλ,
where n=1 for a first order maximum.
Also tanθ=6.09/40.4, giving So, θ= tan^(-1) 0.1507 = 8.572°.
So, d=λ/sinθ=
(650x10⁻⁹)/sin8.572°=
4.36x10⁻⁶m
Converting this to mm, we have d = 4.36x10⁻³mm
Now from formulas, number of lines (N) = 1/d
So, N = 1/d = 1/(4.36x10⁻³) = 229.36 which is approximately 229 lines