# Dispersion by diffraction grating

1. Nov 25, 2009

### Liketothink

1. The problem statement, all variables and given/known data

White light containing wavelengths from 396 nm to 756 nm falls on a grating with 7300 lines/cm. How wide is the first-order spectrum on a screen 2.15 m away?

2. Relevant equations
y=m*L*wavelenth/(d)

3. The attempt at a solution
using the equation I have found y1 to be 62.149 and y2 118.635. I took the difference of the two which gave me 56.486 cm, but that is wrong for some reason. Please help.

2. Nov 25, 2009

### rickz02

Are you sure of that that's the equation for diffraction? Checking the units, it's quite inconsistent... Check the equation again...

3. Nov 25, 2009

### Liketothink

That's the equation I'm used to. If we assume that the angle is small then sintheta=y/L
dsintheta=m*wavelength
d(y/L)=m*wavelength
m*wavelength*L/(d)=y ? I don't know what's wrong with the equation. For the units I coverted everything to cm because 1/7300=d in cm.

4. Nov 25, 2009