Question

In Youngs double slit experiment, the slits are 2mm apart and are illuminated by photons of two wavelengths 1= 12000 and 2= 10000 . At what minimum distance from the common central bright fringe on the screen 2m from the slit will a bright fringe from one interference pattern coincide with a bright fringe from the other ?

• A. 8 mm
• B. 6 mm
• C. 4 mm
• D. 3 mm

Answer 1

The correct option is B 6 mm

d = 2 mm ; D = 2 m

${\lambda }_{1}and{\lambda }_2$ are the two wavelengths

Fringe width for first wave length

$\beta =\frac{\left({\lambda }_1\right)D}{d}$=$1.2mm$

For second wave length

$\beta =\frac{\left({\lambda }_2\right)D}{d}$=$1mm$

At 6 mm distance from center bright fringe 5th5th fringe of first coincides with 6th6th of second.