Question

In Young's double-slit experiment, 30 fringes are obtained in the field of view of the observing telescope, when the wavelength of light used is $4000\mathring{A}$. If we use monochromatic light of wavelength $6000\mathring{A}$, the number of fringes obtained in the same field of view is:

• A. $30$
• B. $45$
• C. $20$
• D. none of these

Answer 1

The correct option is C $20$

Wavelength of light used is inversely proportional to number of fringes observed.

$\therefore {\lambda }_1{n}_1={\lambda }_2{n}_2$

$n_2=\frac{{\lambda }_1{n}_1}{{\lambda }_2}=\frac{4000\times 30}{6000}=20$