Q121 / 170 · Manipal (MET) 2017 · Physics

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

  • A
    $10$
  • B
    $20$
  • C
    $45$
  • Correct answer
    D
    $90$

Solution

Using red light $(\lambda=6600 Å) 60$ 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 $\begin{aligned} \Rightarrow 60 \times \frac{D \lambda}{d} & =n \times \frac{D \lambda^{\prime}}{d} \\ 60 \times \lambda & =n \times \lambda^{\prime} \\ \Rightarrow n=60 \times \frac{\lambda}{\lambda^{\prime}} & =60 \times \frac{6600}{4400}=90 \end{aligned}$

Hence, the correct answer is option: 'D'.

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