[ف.نووي]

Determine the number of grating lines necessary to resolve the 589.59 nm and 589.00 nm sodium lines in second order.
a. 999 b. 680 c. 500
d. 340 e. 380
solution:

1- resolving power must a grating have if these wavelengths are to be distinguished
R=λ/Δλ=((λ_1+λ_2)⁄2)/(λ_2-λ_1 )=(589.30nm)/(589.59nm-589.00nm)=999
2- To resolve these lines in the second-order spectrum
N=R/m=999/2=500