Question

Tenth fringe of wavelength $4000A^o$ coincides with $8th$ fringes of wavelength $\lambda$. Then $\lambda$ is

• A. $50nm$
• B. $555nm$
• C. $450nm$
• D. none

Answer 1

The correct option is D none

Since 10 fringes of ${\lambda }_1$ coincide with 8 fringes of ${\lambda }_2$,

$10{\beta }_1=8{\beta }_2$

$⟹10\times \frac{D{\lambda }_1}{d}=8\times \frac{D{\lambda }_2}{d}$

$⟹\lambda =\frac{10}{8}\times 4000A^{\circ }$

$⟹\lambda =500nm$