Question

A biprism experiment was performed by using red light of wavelength $6500\mathring{A}$ and blue light of wavelength $5200\mathring{A}$. the value of n for which $(n+1{)}^{th}$ blue bright band coincides with $n^{th}$ red band is

• A. $5$
• B. $4$
• C. $3$
• D. $2$

Answer 1

The correct option is B $4$

Formula,

$\beta =\frac{\lambda d}{D}$

given that $n$ fringes of red coinciding with $n+1$ green fringes. Then,

$n{\beta }_r=(n+1){\beta }_g$

$n\times \frac{6500d}{D}=(n+1)\frac{5200d}{D}$

$n\times 65=(n+1)\times 52$

$n=\frac{65-52}{52}$

$\therefore n=4$