Double slit dark fringes

[NelielSwann]

Homework Statement

A blue laser beam of wavelength 470nm (in air) is incident on two narrow slits separated by .2mm and produces an interference pattern on a screen located 2m away from the two slits.

Find the angular position (in degrees) of the 1st dark fringe. Can you use the small angle approximation?

Homework Equations

dsin(theta)=n(lambda)
tan(theta)= y/L

The Attempt at a Solution

I've searched all over for a formula to find the dark fringes, but all i can find is one for the light ones. the dsin(theta)=n(lambda) is used for the maxima. what formula do i use for the minima/dark fringes?

 

[rl.bhat]

For dark fringes use dsin(theta)=(n + 1/2 )(lambda)

 

[jtbell]

For two-slit interference, the minima are halfway in between the maxima.