Flow of liquid in the transparent soap filmZiqing TangAward: Top 100
School: shanghai qibaodwight high school The photo above shows lots of small dynamic irregular shadow patterns in the soap film on a wall. The film of soap was made through the bubble sword and the photo was taken in a dark room with a point source of light in front of it. In this situation, the thickness of the soap film is dynamic and not uniform because of the external gravitational force which leads the soap solution to drain downwards, and the air currents in the room which may cause the thin film to vibrate slightly. The change in the thickness can indicate the flow of liquid in the thin soap film. When light passes through the soap film, some of it is reflected off the surface of the bubble, and some of it is refracted and passes through the bubble. For thinker parts of the bubble film, refracted lights travel through more different path lengths. These variations in path lengths within the thicker section can cause some light rays to refract at slightly different angles upon exiting the film. This leads to a spreading of the light rays originally traveling in the same direction, and the shadow of the bubble will be darker in those areas. Conversely, the lighter shadow is formed at the thinner parts of the bubble film. In this case, the thickness of the transparent bubble turns was shown through the brightness of the shadow. The movements in these shadows show the flow of liquid in the soap film. 
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