Question

Why will one not see an interference pattern for white light in the case of two slits?

Answer 1

White light contains light of different wavelengths. Each wavelength will produce its own interference pattern as it passes through the two slits. The interference pattern will be observed only when the maximum - minimum separation is much larger than the separation between the maxima of the extreme wavelengths in white light (red and violet). Now separation depends upon distace between slits, distance between slit and Screen and wavelength of light. As first two are constant for all wavelengths of white light, the dependence is only upon wavelengths. But different wavelengths of white light are very close in value as they are in nanometer range and thus themaximum - minimum separation is comparable for various wavelenghts.

So,there are so many different wavelengths in white light that the interference pattern from each wavelength overlaps each other and washes out.