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 ${\lambda }^{\prime },n$ fringes are seen, then
$\Rightarrow 60\times \frac{D\lambda }{d}=n\times \frac{D{\lambda }^{\prime }}{d}$
$\Rightarrow 60\times \lambda =n\times {\lambda }^{\prime }$
$\Rightarrow n=60\times \frac{\lambda }{{\lambda }^{\prime }}=60\times \frac{6600}{4400}$
$=90$