Q86 / 180 · AIIMS 2017 · Physics

Two slits are separated by a distance of 0.5 mmand illuminated withlight of wavelength $\lambda=6000 Å$. If the screen is placed at $2.5 \mathrm{~m}$ from the slits. The distance of the third bright fringe from the centre will be

  • A
    $1.5 \mathrm{~mm}$
  • B
    $3 \mathrm{~mm}$
  • C
    $6 \mathrm{~mm}$
  • Correct answer
    D
    $9 \mathrm{~mm}$

Solution

Distance of $n^{\text {th }}$ bright fringe from the centre, $x_n=\frac{n D \lambda}{d}$ So, $\quad \begin{aligned} x_3 & =\frac{3 \times 2.5 \times 6000 \times 10^{-10}}{0.5 \times 10^{-3}} \\ & =9 \times 10^{-3} \mathrm{~m}=9 \mathrm{~mm}\end{aligned}$

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

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