In Young's double-slit experiment, monochromatic light of wavelength $600$ nm is used. The slit separation is $0.5$ mm and the screen is $1.0$ m away. Find the fringe width.

$\beta = \dfrac{\lambda D}{d}$. Convert units to SI before substituting: $\beta = \dfrac{(600 \times 10^{-9}\,\text{m}) \times (1.0\,\text{m})}{0.5 \times 10^{-3}\,\text{m}} = \dfrac{6 \times 10^{-7}}{5 \times 10^{-4}} = 1.2 \times 10^{-3}\,\text{m} = 1.2\,\text{mm}$. A quick sanity check: fringe widths in classroom Young experiments are usually in the millimetre range, so the order of magnitude is right.