Answer
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Work Step by Step
Visible light has wavelengths between 400 and 700 nm. For a thin film like a soap bubble, there are only a few separate visible wavelengths that meet the interference criteria. For example, if the thickness t is 600 nm, then $m\lambda=2t$ only has solutions for wavelengths of 400 nm (m=3) and 600 nm (m=2). This is because the film is very thin, on the order of a few wavelengths.
On the other hand, for a thick film, such as a piece of glass, there are many different wavelengths in the visible range that meet the interference criteria, each with its own m value. For example, if t = 6000000 nm, there are solutions for wavelengths of 400nm (m=15000), 400.027nm (m=14999), …600nm (m=10000), 600.06nm (m=9999), etc. Many, many different colors constructively interfere, and the reflected light, since it contains so many wavelengths, will appear to be white. Individually-colored fringes won’t be noticeable.
