Consider a variety of colors of visible light (say 400 nm to 700 nm) falling onto a pair of slits.
what is the smallest separation (in nanometers) between two slits that will produce a second-order maximum for some visible light?

Explanation:

The relationship between angle and wavelength for maxima and minima in Young's double slit experiment is given by

For constructive interference

For Destructive interference

where

m=order of maxima and minima

for second order maxima i.e.

For smallest separation taking

1/0.14nm = 10^10/1.4 a lot