# I can't figure out what I am doing wrong Getting Frustrated

I can't figure out what I am doing wrong!!! Getting Frustrated!!

1. Homework Statement
A helium-neon laser (λ=629 nm) illuminates a diffraction grating. The distance between the two m=3 bright fringes is 33 cm on a screen 1.72 m behind the grating. What is the spacing between slits of the grating?

2. Homework Equations
Lamda = 629 nm
m = 3
y = 0.33 m
L = 1.72 m
d = ?

3. The Attempt at a Solution
What I did was tan theta = y/L = 0.33/1.72 = 1086 degrees
then I used the equation d*sin theta = m*lamda
and solved for d.... d = m*lamda/sin theta = 1.00x10^-5 m but I am getting it wrong...what am I not doing right???

Related Introductory Physics Homework Help News on Phys.org
dynamicsolo
Homework Helper
If I'm understanding this problem statement correctly, it is giving you the distance between the m=3 maxima on either side of the midline. This means that each of these fringes is half of 10.86º away from the midline, or 5.43º.

So I think this may be the angle you are to use in the diffraction equation, with m = 3 and lambda = 629·10^-9 m.

oh ok thank you thats it!!