Question

Why an excessively thin film of uniform thickness will appear dark in the reflected system?

Answer 1

1. An excessively thin film of uniform thickness will appear dark in the reflected system because in the medium, due to the interference of light waves.
2. Interference of light waves means two or more waves have the same wavelength, frequency, and amplitude, and when they meet together in a medium they enhance the effect of each other.
3. The difference between the two waves of light; is one after getting refracted to the film and reflected back and another one from the top of the thin film.
4. For an excessively thin film, the effective path difference in reflected light $\left(2\mu t\cos r+\frac{\lambda }{2}\right)$ becomes $\frac{\lambda }{2}$, as $2\mu t\cos r$ is eligible in comparison to $\frac{\lambda }{2}$. This is the condition of minimum intensity for all wavelengths, therefore all wavelengths will be absent in the reflected system, hence film will appear dark