Derivation of reflection diffraction grating formula

[JamesGman]

My lecturer has given me this formula for reflective diffraction

m landa = d (cos a - cos b)


where

m is order #
landa is wavelength
d is slit separation
a is angle of inicdence
b is angle of reflection


The problem I have is I wasn't given any justification of this and have no idea where it comes from

 

[neu]

$$\lambda$$ = "lambda"

Do you mean this set up?
http://en.wikipedia.org/wiki/Diffraction#Diffraction_from_an_array_of_narrow_slits_or_a_grating

If so then I am sure your eq simplifies to the wiki eq: $$m \lambda = d sin \theta$$ is the incident angle is zero.

But check this, i haven't