Light incident on a grating for spectroscopy

[KaiserBrandon]

Homework Statement

if light incident on a grating makes an angle phi with the normal of the grating, show that the equation m(lambda)=d(sin(theta)) becomes m(lambda)=d[sin(theta-phi)+sin(phi)]

Homework Equations

m(lambda)=d(sin(theta)) d=spacing of slits, theta=angle displaced from normal of the grating
m(lambda)=d[sin(theta-phi)+sin(phi)]

The Attempt at a Solution

I've been working on trying to get a solution to this equation for quite some time now, and the only thing that makes sense is the equation m(lambda)=d*sin(theta-phi). don't know where +sin(phi) comes from.

 

[Delphi51]

I don't get either of those answers!
Maybe we can put our heads together and sort out our differences!