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In the diffraction pattern, maximum intensity fringes
are obtained when constructive interference takes place,
the condition for which is given by
$$d \sin \theta = n \lambda $$
Where
\(d\) is slit width
\(\theta\) is angle made by light wave, leading to intensity maxima, with horizontal
n = 0, ±1, ±2, ±3… is the order of maxima
\(\lambda\) is wavelength of light
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Wavelength of light can be determined from the fringe width of the diffraction pattern, given by
$$\lambda = \frac{yd}{nD}$$
\(y\) is distance of intensity maxima fringe from central maxima (fringe width)
\(D\) is distance between slit and screen