Question

In Young's double slit experiment using yellow and blue lights of wavelengths $560\text{nm}$ and $420\text{nm}$ respectively, the value of $n$ for which $n^{th}$ order of yellow bright fringe coincides with $(n+1{)}^{th}$ order of blue bright fringe is

• A. $3$
• B. $4$
• C. $2$
• D. $5$

Answer 1

The correct option is A $3$

For bright fringe path difference $=n\lambda$
$(n+1){\lambda }_2=n{\lambda }_1$
$(n+1)420=n560$
$(n+1)3=4n$
$n=3$