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Question 1: A soap bubble looks black when it bursts, why?


A bubble has to burst! This is not strange to any of us.

A bubble remains under the action of four major forces as far as it exists; the surface tension of the soapy liquid, internal pressure of the contained air, atmospheric pressure and the force of gravity. It bursts when there is an imbalance in these forces. Usually, the upper part of the surface becomes very thin by the fluid draining toward the lower part due to gravity and, secondly, due to evaporation of water. Water has more surface tension than detergent (soap etc.), therefore, the thinner part (with reduced number of water molecules due to evaporation) becomes fragile and bursts. However, it is observed that when the thickness reach to considerably less than one-quarter of the wavelength of the incident light, the reflected light from the outer and inner surfaces of the bubble suffer from a destructive interference. All colors are weakened to about same extent and it starts look black against a dark background.

The Physics involved is the bubble surface acts like a thin film. When light falls  on it, part of the ray (yellow in the diagram) is reflected from the outer surface of the bubble and part (green) is refracted into the surface and reflected from the inner surface, having boundary with air inside the bubble. These two parts then interfere with one another giving rise to constructive and destructive interference.
As the part I (yellow) reflects from a denser medium (from the soapy solution), it undergoes a phase change of π/2 = 180 degrees. This means that a crest is reflected as a trough and a trough is reflected as a crest from the bubble outer surface.
The second part (green) which is refracted in the bubble surface encounters a rare medium boundary on the inner surface of the surface. When it is reflected from there, it undergoes no change of phase; a crest reflects as crest and trough as trough. It then refracts in the air, on the same side as incident ray (green again).

The two rays are now interfering with one another and either the condition of constructive or destructive interference will take place between them, depending upon the path difference between them.

Now when the bubble is about to burst, the thickness of the surface is considerably small and it can be said that thickness = 0. When the thickness is zero, this means the path difference between the two rays is zero. At the same time, both the rays are totally anti-phase (because yellow ray has undergone a phase change). Therefore, crest of this ray overlaps with the trough of the green ray and trough of yellow ray overlaps with the crest of green ray. Therefore, both the rays cancel one another and darkness is seen.

Thus the bubble looks black when it bursts.



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