In a Young's double-slit experiment, the interference pattern of bright and dark fringes occurs due to the wave nature of light. The brightness or visibility of the fringes is primarily influenced by two factors: the intensity of the light source and the coherence of the light waves.
Intensity of the light source: The brightness of the fringes is directly related to the intensity of the light source. If the light source used in the experiment is more intense, it means there are a greater number of photons per unit time and per unit area passing through the slits. This increased photon density results in a higher probability of photons hitting the screen at specific locations, leading to brighter fringes.
Coherence of the light waves: Coherence refers to the correlation between the phases of different light waves. In the double-slit experiment, coherent light waves are essential for producing a distinct and well-defined interference pattern. When the waves from the two slits meet on the screen, they interfere constructively at certain points, creating bright fringes, and interfere destructively at other points, producing dark fringes.
To maximize the visibility of the fringes, it is necessary to use a light source that is both intense and coherent. Lasers are commonly employed in Young's double-slit experiments because they provide high-intensity light with a high degree of coherence, resulting in well-defined and easily observable interference patterns.
It's worth noting that factors such as the size of the slits, the distance between the slits and the screen, and the wavelength of light used also play a role in determining the visibility and spacing of the fringes.