In a particular grating the sodium doublet is viewed in third order at to the normal and is barely resolved. We have to find (a) the ruling spacing and (b) the total width of the grating.
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The grating equation is
We calculate first the ruling spacing d using the information that in the third order, , the doublet occurs at to the normal. We thus find that
The other information is that the doublet is barely resolvable in the third order. Using the expression for the resolving power of grating we find the total number of rulings N in the grating.
Therefore, the total width of the grating will be