How Do You Calculate the Width of a Slit in a Diffraction Experiment?

[BuBbLeS01]

Width of Slit...HeLP on setup Please!

Homework Statement

A helium-neon laser (λ=590 nm) illuminates a single slit and is observed on a screen 1.7 m behind the slit. The distance between the first and second minima in the diffraction pattern is 6.78 mm. What is the width (in mm) of the slit?

Homework Equations

The Attempt at a Solution

I think I am supposed to use the equation...

W = 2tL/a
t = wavelength
t = 0.00000059 m
L = 1.7 m
a = .00678 m

but that gets me 0.2956 mm
If I divide that by 2 I get the right answer so I am a little confused on my setup

 

[hage567]

The distance 6.78 mm is the difference between the m=2 and m=1 minima. It is not the distance from the centre of the screen to the second minima (if that's what you are doing). Draw a diagram. You need to find one expression for the distance for m=1 minimum and one for the m=2 minimum. You know the difference between these two is 6.78 mm. This will allow you to solve for the slit width.

 

[BuBbLeS01]

The width of the slit is going from m=1 on 1 side of the central maxima to m=1 on the other side right? And fringe spacing between all fringes is equal correct?