Question

The characteristic yellow light of sodium lamps arises from two prominent wavelengths in its spectrum, at approximately 589.0 nm and 589.6 nm, respectively. The light passes through a double slit and falls on a screen 10 m away. If the slits are separated by a distance of 0.01 mm, how far apart are the two second-order bright fringes on the screen (in mm)? 

Answer 1

The position of the second order bright fringe (constructive interference) is given by

where I have set m = 2 and used the small angle approximation. We also know that

The position of the second order bright fringe is thus