Double-Slit Experiment problem?

[brinstar]

Homework Statement

In a double-slit experiment, the third order bright fringe is 15 mm from the central fringe. What is the distance of the first (zero-th order) dark fringe from the central maximum?

Homework Equations

(m+.5)(lambda) = dsin(theta) => dark fringe
m(lambda) = dsin(theta) => bright fringe

The Attempt at a Solution

I honestly don't know. Like, there's no angle or wavelength. The most I've come up with is 15 mm divided by 3 (because it's the third) and getting 5 mm. I seriously don't know.

 

[lychette]

the equation you need is
dsinθ = nλ for maxima...you have this in your post

 

[brinstar]

the equation you need is
dsinθ = nλ for maxima...you have this in your post

But where do I get the angle and wavelength from?

 

[J Hann]

For angles this small you can use the approximation: sin theta = theta = x / D where x
is the fringe displacement from the central maximum and D is the distance to the screen.