Question

The two slits at a distance of 1 mm are illuminated by the light of wavelength $6.5 \times 10^{- 7}$m. The interference fringes are observed on a screen placed at a distance of 1 m. The distance between third dark fringe and fifth bright fringe will be

• A. 0.65 mm
• B. 1.63 mm
• C. 3.25 mm
• D. 4.88 mm

Answer 1

Answer: (B)

1.63 mm

Answer 2

Distance between n^th bright and m^th dark fringe (n > m) is given as $x = \left( n - m + \frac{1}{2} \right)\beta = \left( n - m + \frac{1}{2} \right)\frac{\lambda D}{d}$

⇒ $x = \left( 5 - 3 + \frac{1}{2} \right) \times \frac{6.5 \times 10^{- 7} \times 1}{1 \times 10^{- 3}} = 1.63mm$.