Question

Monochromatic light from a narrow slit illuminates two narrow slits 0.3 mm apart, producing an interference pattern with bright fringes 1.5 mm apart on a screen 75 cm away. Find the wavelength of the light. Find the fringe width if (a) the distance of the screen is doubled and (b) the separation between the slits is doubled?

• A. 600 nm, 3 mm, 0.75 mm
• B. 400 nm, 1 mm, 0.5 mm
• C. 600 nm, 2 mm, 0.75 mm
• D. 400 nm, 1mm, 2 mm

Answer 1

The correct option is A 600 nm, 3 mm, 0.75 mm

$\lambda =\frac{\beta d}{D}=600nm$, with D doubled, fringe width doubles i.e 3mm and with d doubled, fringe width becomes half i.e 0.75mm.