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## Homework Statement

A quasi-monochromatic beam of light illuminates Young's double-slit setup, generating a fringe pattern having 5.6-mm separation between consecutive dark bands. The distance between the place containing the apertures and the plane of observation is 7 m, and the two slits are separated by 1.0 mm. What is the wavelength of the light?

## Homework Equations

##y_m = (m+1/2)\lambda L / a## (for destructive interference)

## The Attempt at a Solution

##\lambda = \frac {y_m a}{(m+1/2)L}##

Since the separation between consecutive dark bands is 5.6 mm, then we can say that the distance to the first minimum (corresponding to ##m=0##) is ##5.6/2##. Then we have ##a=1## and ##L=7000## and the formula gives ##\lambda=800 nm## but the answer is supposed to be 560 nm. What am I doing wrong?