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There was some discussion in a previous thread ( Why are thin soap bubbles colorless? ) that talked about thin soap bubbles. My question is, what if we go the opposite way? Why are thick bubbles colorless (or less colorful than thin/ "normal" bubbles)? What physical principles come into play in this case?

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Thes question has to be the other way: Why are thin bubbles colorful? A thick material like a window glass is colorless too.

A thin bubble acts the same was like a thin layer of oil at water. If the thickness comes in the range of the wavelength of light, there are intereference effects. There are two reflections from the bubble from both sides. If the thickness is a multiple of half the wavelength, both waves are superimposed and reinforced.

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Here is an image of a soap film which is thinner at the top than at the bottom due to the soap solution moving downwards under the influence of gravity.
The film is viewed using white light.

enter image description here

At the top the film is "balck" because there is no reflected light or rather the reflected light from the two surfaces which are very close together differ in phase by $\pi$ and so interfere destructively.

Lower doen the path length between the two surfaces is such that constructive interference can occur.
You will note that the colour of a fringe at the top is blue and it is red at the bottom because when going down the thickness of the film is getting larger and blue light which has a shorter wavelength than red light needs a thinner thickness of film to exhibit constructive interference.

However, the fringes lower down become less distinct because the thickness for the constructive interference for one wavelength is such that destructive interference occurs for another wavelength - there is no evidence of distructive interference after two fringes.

This overlap of fringes for different wavelength becomes greater and greater as the thickness of the film gets larger and larger until eventually no fringe pattern is seen and the light which is reflected is "white".

Here is an image to show what would happen if only three wavelengths were present.

enter image description here

The other parameter which comes into play is the "flatness" of the surfaces of the object which is doing the reflecting.
Even a small deviation from flatness of a few wavelengths will affect the interference pattern that is observed.

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