# Homework Help: Diffraction grating

1. Mar 25, 2004

### lovelylm1980

White light containing wavelengths from 405 nm to 760 nm falls on a grating with 7510 lines/ cm. How wide is the first-order spectrum on a screen 2.29 m away?

Is this correct

sin theta= (1)*(4.05e-7m)/(1.33e-6m)= 0.305 theta=17.8
location= 2x(tan 17.8)= (2*2.29)*0.321=1.47m

sin theta= (1)*(7.60e-7m)/(1.33e-6m)= 0.571 theta=34.8
location= 2x(tan 34.8)= (2*2.29)*0.695= 3.18m

so then should the answer to the question be
3.18m+1.47m/2= 2.33m

2. Mar 25, 2004

### Staff: Mentor

Why did you multiply by 2?

3. Mar 25, 2004

### lovelylm1980

so then should i multiply the angle by 2

4. Mar 25, 2004

### Staff: Mentor

Why are you multiplying anything by 2??

5. Mar 25, 2004

### lovelylm1980

because the book states that with large angles you should multiply by two either way I don't get the correct answer based on the formula i used. Whether I multiply by 2 or not the answer still seems to be wrong. Am I not supposed to add at the end then divide by two for the answer?

6. Mar 25, 2004

### Staff: Mentor

That makes no sense.

Find the location of the first order maxima for each wavelength. Subtract those two numbers and you'll have the width of the first order spectrum.