Monochromatic light falls on a screen 1.60 m from two slits separated by 2.11 mm. The first- and second-order bright fringes are separated by 0.550 mm. What is the wavelength of the light?

725 x10^-9m

Explanation:

We know that the optical path difference is (ax)/D

Where

a=2.11mm

D=1.60m

x= position on the screen

the optical path differences varies of lambda = wavelength

That is BTW two bright fringes

So therefore

x2-x1=lambda x D/a

lambda = ( x2 - x1) x a/D = 0.55 x 10^-3 x 2.11E-3 / 1.60 = 725nm

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.