Colours of a soap film
Encyclopedia
The iridescent colours of soap bubbles are caused by interfering of (internally and externally) reflected light waves and are determined by the thickness of the film. This phenomenon is not the same as the origin of rainbow
colours (caused by the refraction
of internally reflected light), but rather are the same as the phenomenon causing the colours in an oil slick on a wet road.
As light impinges on the film
The total reflection observed is determined by the interference of all these reflections. Since each traversal of the film causes a phase shift
proportional to the thickness of the film and inversely proportional to the wavelength, the result of the interference depends on these two quantities. So at a given thickness, interference is constructive for some wavelengths and destructive for others, so that white light impinging on the film is reflected with a hue
that changes with thickness.
A change in colour can be observed while the bubble is thinning due to evaporation and draining. Thicker walls cancel out red (longer) wavelengths, causing a blue-green reflection. Later, thinner walls will cancel out yellow (leaving blue light), then green (leaving magenta
), then blue (leaving a golden yellow). Finally, when the bubble's wall becomes much thinner than the wavelength of visible light, all the waves in the visible region cancel each other out and no reflection is visible at all. When this state is observed, the wall is thinner than about 25 nanometers, and is probably about to pop. This phenomenon is very useful when making or manipulating bubbles as it gives an indication of the bubble's fragility.
Interference effects also depend upon the angle at which the light strikes the film, an effect called iridescence
. So, even if the wall of the bubble were of uniform thickness, one would still see variations of colour due to curvature and/or movement. However, the thickness of the wall is continuously changing as gravity pulls the liquid downwards, so bands of colours that move downwards are usually observed.
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
colours (caused by the refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
of internally reflected light), but rather are the same as the phenomenon causing the colours in an oil slick on a wet road.
As light impinges on the film
- some of it reflectsReflection (physics)Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
off of the outer surface - some of it enters the film and reemerges after reflecting off the second surface
- some of it enters the film and reemerges after bouncing back and forth between the two surfaces from 1 to n times
The total reflection observed is determined by the interference of all these reflections. Since each traversal of the film causes a phase shift
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
proportional to the thickness of the film and inversely proportional to the wavelength, the result of the interference depends on these two quantities. So at a given thickness, interference is constructive for some wavelengths and destructive for others, so that white light impinging on the film is reflected with a hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
that changes with thickness.
A change in colour can be observed while the bubble is thinning due to evaporation and draining. Thicker walls cancel out red (longer) wavelengths, causing a blue-green reflection. Later, thinner walls will cancel out yellow (leaving blue light), then green (leaving magenta
Magenta
Magenta is a color evoked by light stronger in blue and red wavelengths than in yellowish-green wavelengths . In light experiments, magenta can be produced by removing the lime-green wavelengths from white light...
), then blue (leaving a golden yellow). Finally, when the bubble's wall becomes much thinner than the wavelength of visible light, all the waves in the visible region cancel each other out and no reflection is visible at all. When this state is observed, the wall is thinner than about 25 nanometers, and is probably about to pop. This phenomenon is very useful when making or manipulating bubbles as it gives an indication of the bubble's fragility.
Interference effects also depend upon the angle at which the light strikes the film, an effect called iridescence
Iridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
. So, even if the wall of the bubble were of uniform thickness, one would still see variations of colour due to curvature and/or movement. However, the thickness of the wall is continuously changing as gravity pulls the liquid downwards, so bands of colours that move downwards are usually observed.