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It is given that Lambda=630nm=630 x 10^-9 m
It is given that the grating is 450 Lines per 1mm
First, we need a, which is distance between 2 lines only.
To get a, we need to use a=d/n, where d is the whole length of grating and n is number of lines.
We need to change 1 mm into m, which is 0.001 m.
Now, a= 0.001/450=2.2 x 10^-6m
We now need to use, asintheta=nLambda where a is separation between 2 lines.
We need to find n, we put theta=90, so (2.2 x 10^-6)sin90=n(630 x 10^-9), notice that all values should be the same unit. I prefer changing them all to meters.
Notice that our answer of n=3.52, but n has to be a whole number, so we round it down to 3.
I hope I helped you, i tried to be as clear as possible.
