A long narrow horizontal slit is placed 1 mm above a horizontal plane mirror. The interference between the light coming directly from the slit and that after reflection is seen on a screen 1.0 m away from the slit. Find the fringe-width if the light used has a wavelength of 700 nm.
The distance between the screen and the slit, 
Wavelength of light, 
Here, we assume that the second slit is actually the virtual image of real slit.
Therefore, separation between the slits 
Therefore, fringe width is 
1