Pathlength Difference from two waves

[skibum143]

Homework Statement

Two slits separated by a distance of d = 0.12mm are located at a distance of D = 780mm from a screen. The screen is oriented parallel to the plane of the slits. The slits are illuminated by a coherent light source with a wavelength of lambda = 549E-6 mm. The interference pattern shows a peak at the center of the screen (m=0) and then alternating minima and maxima.
7. [1pt]
What is the pathlength difference between the two waves from the two slits at the first (m=1) maximum on the screen?

Homework Equations

d*sintheta = m*wavelength
sintheta = y(dist bt two maxima) / D(distance to screen)

The Attempt at a Solution

I'm really confused about what the "pathlength difference" refers to. It doesn't specify it in the book. I know the angles take different paths to get to the same point because of the distance inbetween them, but is it asking for an angle?
I solved the problem for y using the above equations, and at the first maxima (m=1), y(distance between the two maxima) is 0.0036m. But I don't know where to go from there?

 

[skibum143]

So...I think I just realized that pathlength difference refers to "m", but what are the units??

 

[skibum143]

Ah, just figured it out...you have to use "m" and multiply it by the wavelength.