Diffraction Grating: Calculate Wavelengths

[SGL18]

Homework Statement

light from an atomic hydrogen lamp falls on a diffraction grating with 200 lines per centimetre. on a screen 2.5 m away, the distance between green spots of the first and second order spectra is 2.43cm while the distance between a green spot and a red spot both of the first order spectrum is 8.5mm. calculate the wavelengths of the green and red radiation?

Homework Equations

λ = d sinQ = md/D & d=1/N

The Attempt at a Solution

Not sure about the 1st and 2nd order spectrum stuff! i know the equation will end up with
(a) x(b)/2.5 but I am not sure how to get the values of (a) and (b)?

 

[nickjer]

The first and second order spectrum has to do with the 'm' in the diffraction grating equation. I have no idea what happened to your 'm' on the left. The equation is:

m\lambda = d \sin(\theta_m)