Young's Double Slit Interference

Young's double slit experiment, treated in NCERT, produces bright and dark fringes whose spacing, the fringe width, is $\beta =\frac{\lambda D}{d}$, where $\lambda$ is the wavelength, $D$ the slit-to-screen distance, and $d$ the slit separation. Converting to consistent SI units, $\lambda =600\times 10^{-9}$ m, $D=2$ m, and $d=1\times 10^{-3}$ m. Substituting gives $\beta =\frac{(600\times 10^{-9})(2)}{1\times 10^{-3}}=\frac{1200\times 10^{-9}}{10^{-3}}=1.2\times 10^{-3}$ m, which is 1.2 mm. The value 0.6 mm is wrong because it omits the factor of two from the 2 m screen distance. The value 2.4 mm is wrong because it doubles the wavelength or the distance erroneously. The value 0.3 mm is wrong because it divides rather than multiplies by the screen distance. As stated in NCERT Class 12, Chapter 10 (Wave Optics), fringe width grows with wavelength and screen distance but shrinks with wider slit separation. A magnitude check confirms that visible-light fringes at metre distances are of order a millimetre, matching the 1.2 mm result.