If the monochromatic source in Young's double slit experiment is replaced by white light, then

In Young’s double‑slit experiment the condition for a bright (constructive) fringe is

where $d$ is the slit separation, $\theta$ the angle of observation, $m$ an integer (order), and $\lambda$ the wavelength of the light.

When the source is monochromatic (single $\lambda$), every order $m$ produces a bright fringe at the same angle for all photons, giving a regular pattern of equally spaced bright and dark fringes.

If the source is replaced by white light, it contains many wavelengths simultaneously (e.g., red, green, blue). For a given order $m$ each wavelength satisfies the interference condition at a slightly different angle because the right‑hand side $m \lambda$ varies with $\lambda$. Consequently, the higher‑order fringes split into separate coloured bands, each colour appearing at the angle appropriate for its wavelength.

At the central position ($\theta = 0$), the path difference between the two slits is zero for all wavelengths ($d sin0 = 0$). Hence all colours overlap exactly, producing a bright white fringe at the centre.

Moving away from the centre, the overlap is lost: red light, having the longest wavelength, deviates the most, while blue deviates the least. This creates a few coloured fringes on either side of the central white fringe, and the pattern quickly fades because the different colours no longer coincide.

Therefore, with white‑light illumination the observed pattern consists of a central bright white fringe surrounded by a few coloured fringes.