Question

In a Fresnel's bi-prism experiment, the fringe of width $0.05mm$ is observed on a screen at a distance of $1.5m$ from the source . When a convex lens is placed between the source and the screen, for two positions of the lens image of interfering sources are produced on the screen. The separation between the two images being $0.04$ and $0.01mm$, respectively. The wavelength of light used is

• A. $6.67nm$
• B. $0.667nm$
• C. $667nm$
• D. $667A^o$

Answer 1

The correct option is B $0.667nm$

Distance between slits, $d=\sqrt{d_1{d}_2}=\mathrm{0..02}mm$

Fringe width, $\beta =\frac{D\lambda }{d}$
hence $\lambda =\frac{\beta d}{D}=0.667nm$