A soap film of thickness 0.0011 mm appears dark when seen by the reflected light of wavelength 580 nm. What is the index of refraction of the soap solution, if it is known to be between 1.2 and 1.5?

Given, thickness of the film

Wavelength of light, .

Let be the refractive index.

Here, the bubble appears dark due to destructive interference.

For substituting various values of n, we get for n=5.

Hence, as it has to lie between 1.2 and 1.5.