Final answer:
The reflected ray is faint when you use a white screen because the interference fringes for light reflected from a thick piece of glass are more difficult to see than those from a thin film. The use of monochromatic light would make the interference pattern more distinct.
Step-by-step explanation:
When light strikes a thin film, such as soap bubble or an oil slick, it is partially reflected and partially refracted at the top surface. The refracted ray is then partially reflected at the bottom surface and emerges as a second ray. These two rays interfere with each other, resulting in interference fringes or colors. The visibility of these fringes depends on the thickness of the film and the indices of refraction of the media involved.
In the case of a thick piece of glass, the difference in path length between the reflected and refracted rays is much larger compared to that of a thin film. This leads to a greater phase difference and a more pronounced interference pattern, making the fringes easier to see. If monochromatic light is used, the interference pattern would be even more distinct, as there would be no overlapping of colors.