In Young's double-slit experiment, when monochromatic light (light of a single wavelength) is used, the interference fringes observed on the screen are typically bright and dark bands. However, when sunlight (which consists of a broad range of wavelengths) is used instead, the interference fringes become colored. This phenomenon is known as "chromatic dispersion" or "dispersive coloration."
The reason for the colored interference fringes is that different wavelengths of light experience different amounts of diffraction and interference as they pass through the double slits and interact with each other. Each wavelength of light corresponds to a specific color, and these different colors separate due to their different diffraction properties.
When sunlight passes through the double slits, the different wavelengths of light separate and interfere with each other, creating a pattern of colored fringes. This separation is a result of the phenomenon of dispersion, where different wavelengths of light refract or bend by different amounts when passing through a medium (in this case, the double slits).
The interference pattern produced by the overlapping colors leads to a series of colored fringes on the screen, with different colors corresponding to different positions of constructive and destructive interference.
It's worth noting that this dispersion effect occurs because sunlight contains a wide range of wavelengths. If only a narrow range of wavelengths within the sunlight spectrum is selected (e.g., using a narrowband filter), the interference fringes will appear less colored and resemble the typical bright and dark bands observed with monochromatic light.
In summary, the appearance of colored interference fringes when sunlight is used in Young's double-slit experiment is due to the dispersion of different wavelengths of light, resulting in the separation and interference of different colors.