In Young's double slit experiment, the fringe width is given by:

where:
is the wavelength of light,
is the distance from the slits to the screen, is the separation between the slits.

Here's how the change affects the fringe width:

Initial Situation:

When , the fringe width is:

After Increasing Slit Separation:
When is increased to 0.4 mm:

Comparing the Two Fringe Widths:
Notice that:

This means the fringe width is halved, which is a reduction of .

Thus, the fringe width decreases by when the slit separation is increased from 0.2 mm to 0.4 mm.