# Homework Help: Diffraction orders seen

1. Jul 21, 2010

### clockworks204

1. Light of wavelength 632.8nm is normally incident upon a grating of 5000 lines/cm. How many different diffraction orders can be seen in transmission?

2. m(lambda)=d(sintheta)

3. According to my text, d=1/5000lines/cm = 2x10^-6m
and sintheta= lambda/d = 632.8x10^-9/ 2x10^6 = .3164
If I take sin^-1(.3164), I get 18 degrees.
From here, I don't know what to do because the answer is that 7 beams exist, corresponding to 4 orders, and I don't know how to get to that answer!

2. Jul 21, 2010

### rl.bhat

sinθ = mλ/d.

λ/d = 0.3164. sinθ cannot be more than 1. So m should be 3. Hence there are 3 beams on either side and one central beam.

3. Jul 22, 2010

### clockworks204

Thanks for your reply. Although I understand what you are saying about the 3 beams on each side with one central, how do you derive that explanation from 0.3164?

4. Jul 22, 2010

### rl.bhat

sinθ = nλ/d. if n = 3
sinθ = 3*λ/d = 3*0.3164 = 0.9492< 1. So n cannot be more than 3.

5. Jul 22, 2010

### clockworks204

Thanks, I understand now.