Question

The interference pattern shifts downward by 4mm,
when a polystyrene sheet of refractive index 1.8
is place in front of the lower slit. If wavelength used
is a = 683 nm, separation between slits is 2 mm
and separation between slits and screen is 10 cm
then thickness of sheet
(1) 0.1 mm
(2) 0.2 mm
(3) 0.3 mm
(4) 0.4 mm
The reflected ray is completelu nola​

Answer 1

The thickness of the polystyrene sheet that makes the interference pattern shift downwards by 4 mm is equal to 0.1mm.

• Optical path length for the wave passing through the film will be equal to μt.
• t=y×d/(μ-1)×D ; where t is the thickness of the sheet, y is the shift in the fringe,d is the fringe width and D is the separation between the slit and the screen.
• Now, substituting the values we get the thickness of sheet to be equal to 0.1 mm