Question

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

Answer 1

Wavelength $\lambda$ is given by,

$\beta =\frac{\lambda D}{d}$

a)

the distance of the screen is doubled

$\beta =2\times 1.5mm=3mm$

b)

the separation between the slits is doubled

$\beta =\frac{1.5}{2}mm=0.75mm$