Solution

Given,

Distance between two slits (d) = 0.3 mm = 0.3 x 10^-3 m,
Distance between slits and the screen (D) = 60 cm = 0.6 m,
Wavelength of light (λ) = 600 mm = 600 x 10^-9 m = 6 x 10^-7 m,
Fringe width (β) = ?

We have,

\(\beta=\frac{\lambda D}d=\frac{6\times10^{-7}\times0.6}{0.3\times10^{-3}}\)
∴ \(\beta=1.2\times10^{-3}\) m

Hence, the fringe width is 1.2 x 10^3 m.