# Optics/Spectrum Question

1. May 31, 2006

### Arowana

Hi, i exam is coming up in a week and i have no clue so wat ever on how to solve this question.
How do i calculate the minimum number of lines???

What i need to answer to have full marks??

Q:

The Sodium vapor spectrum contains two bright yellow lines which have almost the same
wavelength (589.0 nm and 589.6 nm). Calculate the minimum number of lines required
in a grating which can resolve these lines in the second order.
:(

2. May 31, 2006

### Pietjuh

You know that the resolving power is given by $$R = \frac{\lambda}{\Delta\lambda}$$. But we also have the diffraction law which states $$a\sin{\theta} = m(\lambda + d\lambda)$$ so $$R = m N$$ where m is the order of the diffraction and N the total number of slits in the grating. So what you want to do is to first calculate the resolving power, and then just divide by the diffraction order to obtain N

3. May 31, 2006

### Gokul43201

Staff Emeritus
Arowana, you are supposed to show your effort before asking for help !

Pietjuh, please require that students asking for help first show what they've tried.

4. May 31, 2006

### Arowana

THanks Pietjuh , and sorry for getting you in trouble.

5. Jun 8, 2006

### IrAlien

Arowana, try looking at your laboratory manual Experiment 44, Diffraction Grating.