Interference was observed in a chamber where air was present. Now, the chamber is evacuated and the same light is used. A careful observer will note:

interference in which width of the fringe will be slightly increased

The correct answer is Option (4) → interference in which width of the fringe will be slightly increased

The fringe width (β) is given by,

$β=\frac{λD}{d}$

When the chamber is evaluated, the refractive index (R) decrease to 1, causing the wavelength to increase.

Since the fringe width is proportional to the wavelength (λ), the fringe width will increase.