Question

Would a simple or complex interference pattern appear if, instead of laser light, it was white light shining through the double-slit?

Answer 1

If the Laser light (monochromatic light) is replaced by white light, then we get a multicolored fringe pattern on the screen.

The reason is as stated below

At the center, the difference in path lengths being zero gives zero phase difference for all wavelengths. Hence the center band will be white. But at other points on the screen, the phase difference is given by

where

is the difference in path lengths.

Since wavelength is different for different colors, the fringe width will be different for different color and hence all other fringes will be colored. In that sense, the interference pattern will be complex if we use white light.