When a source of white light is used instead of a monochromatic source in a Young's double-slit experiment, several changes occur in the interference fringes:
Broadening of fringes: White light consists of a mixture of different wavelengths (colors). Each wavelength creates its own set of interference fringes. As a result, the fringes become broader and overlap with each other, creating a blur or smearing effect. This happens because each color interferes differently, leading to a combination of overlapping fringe patterns on the screen.
Color dispersion: White light contains a range of colors with different wavelengths. When passing through the double slits, these different wavelengths are diffracted by varying amounts, causing them to spread out. This dispersion leads to the separation of colors within the interference pattern. You may observe a sequence of colors along the fringes, with the longer wavelengths (such as red) being deviated less and appearing closer to the center, while the shorter wavelengths (such as violet) are deviated more and appear farther away from the center.
Central maximum intensity: The central bright fringe, known as the central maximum, tends to be more intense and whitish compared to the surrounding fringes. This is because light from all the different wavelengths constructively interferes at the center, resulting in a brighter white region.
Reduced visibility: Due to the broadening and overlap of fringes, the visibility of the interference pattern decreases. Visibility refers to the sharpness and distinctness of the bright and dark fringes. With white light, the interference pattern becomes less distinct, and it may be more challenging to observe clear and well-defined fringes.
Overall, using white light in a double-slit experiment introduces additional complexities and characteristics due to the range of wavelengths present. It results in a blurred, dispersed, and less distinct interference pattern compared to the sharp and well-defined pattern observed with monochromatic (single wavelength) light.