Question

In an interference experiment, third bright fringe is obtained at a point on the screen with a light of $700\text{nm}$. What should be the wavelength of the light in order to obtain $5^{th}$ bright fringe at the same point?

• A. $500\text{nm}$
• B. $630\text{nm}$
• C. $750\text{nm}$
• D. $420\text{nm}$

Answer 1

The correct option is D $420\text{nm}$

At same spot, fringes coincide if
$n_1{\lambda }_1=n_2{\lambda }_2$
$\Rightarrow 3\times 700=5\times {\lambda }_2$
$\Rightarrow {\lambda }_2=420\text{nm}$