Question

Interference was observed in an interference chamber when air was present. Now, the chamber is evacuated and if the same light is used, a careful observation will show:

• A. no interference
• B. interference with bright band
• C. interference with dark bands
• D. interference in which breadth of the fringe will be slightly increased

Answer 1

Answer: (D)

interference in which breadth of the fringe will be slightly increased

Answer 2

When chamber is evacuated, the refractive index $(\mu)$ decreases. Therefore, wavelength $\left(\lambda \propto \frac{1}{\mu}\right)$ increases. Now, fringe width is given by $\beta=\frac{\lambda D}{d}$ $\Rightarrow \beta \propto \lambda$ Therefore, as wavelength increases, the fringe width will increase.