Effect of replacing the monochromatic source with white light on the interference fringes:

When a monochromatic source of light is used in the double-slit interference experiment, a clear pattern of bright and dark fringes is observed due to the constructive and destructive interference of light waves. However, if the monochromatic source is replaced with a source of white light, several changes occur in the interference pattern.

1. Appearance of colors:
When white light is used, the interference pattern no longer consists of just bright and dark fringes. Instead, a series of colored fringes are observed. This is because white light is made up of a range of wavelengths, each of which has a different color. These different colors undergo different amounts of deviation and interference, resulting in a colorful pattern.

2. Disappearance of interference pattern:
The appearance of colors in the interference pattern is due to the phenomenon of dispersion. Each wavelength of light is diffracted differently and produces its own set of fringes. As a result, the distinct interference pattern with bright and dark fringes is no longer visible. Instead, a broad spectrum of colors is observed.

3. Fringe closest to the central fringe is red:
When white light is used, the fringe closest to the central fringe is typically red in color. This is because red light has the longest wavelength among the visible spectrum, and it is diffracted the least. Therefore, it appears closest to the central fringe.

4. Change in fringe separation:
The fringe separation, which is the distance between adjacent fringes, also changes when white light is used. Each color of light experiences a different amount of diffraction and interference, leading to variations in the fringe separation. As a result, the fringe separation increases compared to the case of monochromatic light.

Overall, replacing the monochromatic source with white light in the double-slit interference experiment results in the disappearance of the interference pattern, the appearance of colored fringes, a red fringe closest to the central fringe, and an increase in fringe separation. These changes are due to the phenomenon of dispersion, where different wavelengths of light are diffracted at different angles, leading to a broad spectrum of colors.

The interference patterns due to different component colours of white light overlap (incoherently). The central bright fringes for different colours are at same position. Therefore, the central fringe is white.