How wide is the base of a trihedral prism which has the same resolving power as a diffraaction grating with `10000` lines in the second order of the spectrum if `|dn//d lambda| = 0.10mu m^(-1)`?

Explanation

`|(dn)/(d lambda)| = kN = 2 xx 10,000`
`b xx 0.10 mu m^(-1) = 2 xx 10^(4)`
`b= 2xx 10^(5)mum = 0.2m = 20cm`.