Answer 1

The fringe width in Young's double slit experiment depends on the separation of the slits.

\[\chi = \frac{\lambda D}{d}\]

where

\[\lambda =\text{ wavelength}\]

\[\chi =\text{ fringe width}\]

\[D =\text{ distance between slits and screen}\]

\[d =\text{ separation between slits}\]

On increasing d, fringe width decreases. If the separation is increased too much, the fringes will merge with each other and the fringe pattern won't be detectable.