Monochromatic light falling on two slits 0.042 mm apart produces the fifth order fringe at a 7.8o angle. What is the wavelength of the light used?

The fringes are the constructive interference which occurs at

nl = dsinq

Where n is the order, in this case, 5, so we have:

5l = dsinq

l = dsinq/5

l = (.042E-3 m)(sin(7.8o)/5) = 1.1x10-6 m = 1.1 mm