Question

In Young's experiment, using red light $(\lambda=6600\,\mathring{A}), 60$ fringes are seen in the field of view. How many fringes will be seen by using violet light $(\lambda=4400\,\mathring{A})$ ?

• A. 10
• B. 20
• C. 45
• D. 90

Answer 1

Answer: (D)

Using red light $(\lambda=6600\,\mathring{A}) 60$ fringes are seen.
Hence, range of field of view is $60 \times w=60 \times \frac{D \lambda}{d}$
Using light of wavelength $\lambda', n$ fringes are seen, then
$\Rightarrow 60 \times \frac{D \lambda}{d}=n \times \frac{D \lambda'}{d}$
$\Rightarrow 60 \times \lambda=n \times \lambda'$
$\Rightarrow n=60 \times \frac{\lambda}{\lambda'}=60 \times \frac{6600}{4400}$
$=90$