Question:
A long narrow horizontal slit is placed $1 \mathrm{~mm}$ above a horizontal plane mirror. The interference between the light coming directly from the slit and that after reflection is seen on a screen $1.0 \mathrm{~m}$ away from the slit. Find the fringe-width if the light used has a wavelength of $700 \mathrm{~nm}$.
Solution:
