How Do You Calculate the Slit Spacing in a Double-Slit Experiment?

[BuBbLeS01]

Homework Statement

Light from a helium-neon laser (λ=590 nm) is used to illuminate two narrow slits. The interference pattern is observed on a screen 3.42 m behind the slits. Twelve bright fringes are seen, spanning a distance of 47 mm. What is the spacing (in mm) between the slits?

Homework Equations

The Attempt at a Solution

t = wavelength

t= 590 nm
L = 3.42 m
y = 47 mm
12 fringes
so the space between 2 fringes would be 47mm/11 = 4.27 mm

d = tL/y = 4.73 x 10^-4 m

I don't have the answer to this problem so is this the right way to do this?

 

[BuBbLeS01]

Does anyone know if this is right?

 

[A_lilah]

Does anyone know if this is right?

your variables are a bit confusin. try labeling them. also, are you saying that the light is coming straight out? (because light doesn't usually stay in a straight beam, does it)