Soap bubble film is very thin, only on the order of micron or submicron. When light shines on a thin film, there are two processes: transmission and reflection. Interference is related to reflection. When the remaining light enters the film, the first reflection of light occurs on the first surface and the second reflection occurs on the second surface. As a result, the two reflected lights overlap and interfere, forming light with a new wavelength. When the film meets certain conditions, two kinds of reflected light with a certain wavelength will cancel each other, and the complementary color of the cancelled light will appear in the film. Because of gravity, the thickness of the film at different positions is different and the wavelength of interference is different, so we will see different complementary light colors at different positions.
The reason why soap bubbles become colored:
The soap film itself is colorless, just like transparent cellophane, and sunlight will be reflected on both sides of the soap film. Sunlight passes through the front and meets the back, and is immediately reflected; When the reflected light goes back to the front, it will cause some reflection. Since sunlight is composed of seven monochromatic colors, if the two reflected red lights cancel each other out somewhere in the soap film, what you see here is sunlight without red light, which looks blue-green. On the other hand, the light of one color is enhanced to show another color. This is how soap bubbles break down sunlight and present colorful colors.
In fact, this phenomenon is not only caused by soap bubbles, but also caused by light penetrating any transparent film. For example, we often see the oil film on the water surface, the wings of dragonflies or flies, CD films and so on. , in the sunlight will appear colorful. This is the same as the color of soap bubbles. This is because sunlight is composed of seven colors: red, orange, yellow, green, cyan, blue and purple. When it reflects back and forth on the front and back sides of the film, because the distance between the two sides is very small, the two beams of light reflected from the front and back sides may overlap, so that some of the seven colors of sunlight will be strengthened in different thicknesses, while others will be weakened or even offset each other. As a result, some places on the film are red, some places are blue, and some places are other colors, so we see colorful. Also called film color. This phenomenon is called light interference. So this is the ghost of the sun! Nature really contains many mysteries, waiting for us to explore!
Reasons for the constant change of soap bubbles:
In fact, all this stems from an optical phenomenon called "thin film interference". We usually use detergents (detergent, washing powder, etc. ) mixed with water in a certain proportion to blow soap bubbles, so the foam is a thin water film wrapped by detergent molecules. So, what happens when light hits this thin layer of water?
Air and water are light-transmitting media, and their junction is called "interface". When the light meets the interface, part of it will be reflected back, and part will continue to move forward and refract. For example, if we insert a chopstick into the water, if we look at it from the side, we will find that the chopsticks in the water are bent, which is refraction; And when we look down at the water, if the angle is right, we can also see the mirror image of chopsticks, which is reflection. This thin water film forming soap bubbles has two "air-water" interfaces: when light from the air meets the water film, that is, the first "air-water" interface, part of it is reflected and the other part is refracted; The refracted part passes through the water film and meets the second "air-water" interface. At this time, part of the light is reflected back, while the other part of the light passes through it and returns to the air. In this way, some light will be reflected many times before leaving the water film.
Light has an attribute: multiple light waves in the same place will superimpose their own intensities. This phenomenon is called "interference", which is an effect of "stronger in-phase and counteracting in opposite phases": if the peak of one beam of light is superimposed with the peak of another beam of light, the result will become stronger; And if the peak of one beam of light and the trough of another beam of light are superimposed together, the results will cancel each other out. These two phenomena are called constructive interference and destructive interference respectively. The light reflected back and forth in the soap bubble film will interfere. But generally speaking, complete constructive interference and complete destructive interference are two extremes, and the actual interference situation will be somewhere in between. Moreover, the actual interference is related to the wavelength of light (that is, color) and the thickness of the film, which weakens the reflected light of some colors and enhances the reflected light of some colors. As a result, the film produces a changing color instead of "white" sunlight.
When the soap bubble is blown out, it is not a solid, but the thickness of the foam keeps changing. Under the action of gravity, the water at the top of the bubble will flow to both sides, causing the top of the bubble to become thinner than the bottom; At the same time, the water in the bubbles will evaporate. Generally speaking, bubbles will gradually become thinner after being blown out, and finally they will be too thin to withstand the external pressure impact and rupture. In the process of soap bubble thinning, the interference of the film will also change, which will make the surface of soap bubble show a changing color.
Colorful soap bubbles can not only be appreciated, but also the thickness of bubbles can be inferred from their colors. The results of computer simulation show that the relationship between the color of reflected light on the bubble surface, the incident angle of incident light and the film thickness is also like a gorgeous rainbow.
Thick films, their reflection colors basically alternate between light red and green. However, when the film thickness drops below 500nm, more colors begin to appear and the colors will become more vivid. For example, when the film thickness is between 100nm and 450nm, bright gold and blue will appear. This thickness is almost only 1% of the hair diameter. If it is thinner, the film will gradually darken and become gray-black. This is because the film is too thin to effectively interfere with visible light at this time, and most of the light is "lost" through the film, so there is basically no reflected light, which is grayish black. At this time, the soap bubble is not far from bursting.
So, when you see the bright gold and blue stripes on the foam, you know it is going to fail. If you want to blow more bubbles in the bubble, you'd better poke it in from the light red and light green places, because only there is a thick bubble film that can withstand poking.
The phenomenon of membrane interference does not only appear on soap bubbles. Any transparent film whose thickness is close to the wavelength of visible light will produce this kind of film interference under suitable conditions. For example, the oil film on the pond will also show rainbow color; Camera lenses, anti-blue glasses lenses, and burned blade surfaces will also show special colors. Behind these colors is actually the effect of film interference.
Nowadays, there are many soap bubble lovers in the world. They set up global organizations such as the International Bubble Artists Association, devoted themselves to exploring the secret of soapy water blowing bigger bubbles, and even constantly set a new Guinness World Record for bubble size, responding to childhood longing and yearning for big bubbles.
Small soap bubbles hide the secrets of nature. Physicists can study surface tension, mathematicians can study the smallest surface, biologists can study the biochemical mechanism of thin films in organisms, and mechanics can study the inflatable structure of thin films through them ... Soap bubbles have made great achievements in scientific research, giving scientists a lot of inspiration.
For example, in the early 20th century, when studying the torsion of an elastic cylinder, the German scholar Prandtl found that the equation satisfied by the internal stress function in the cylinder was the same as that satisfied by the thin film under its own weight. Not long ago, Hamid Kelly, an expert in fluid mechanics at Bordeaux University in France, regarded soap bubbles as an ideal model for studying atmospheric phenomena and put forward his own conjecture about the general law of cyclone evolution. Even the paper "Observation on the Branch Flow of Light" based on soap bubbles appeared on the cover of Nature, proving that soap bubbles still have many secrets worth exploring.
As Kelvin, a famous British physicist, said, "Blow a soap bubble, observe it, and you will spend your whole life studying it, which will lead to one physics lesson after another." The wonder and fun of nature are worth exploring all our lives.
Origin of soap in China:
Soap is a synthetic detergent that appeared in Song Dynasty. It is made by mashing natural Gleditsia sinensis (also known as Gleditsia sinensis, hanging knife and Gleditsia sinensis, commonly known as Gleditsia sinensis) and adding spices to make an orange-sized ball, which is specially used for washing face and bathing body, commonly known as "soap ball". Song people's careful "Old Wulin Stories" (Volume 6) and "Little Broker" recorded that there was a businessman who specialized in "soap troupe" in Lin 'an, Kyoto in the Southern Song Dynasty.
Li Shizhen's Compendium of Materia Medica in the Ming Dynasty recorded the making method of Gleditsia sinensis: Gleditsia sinensis was born in a high mountain, with tall trees, leaves like sandalwood Gleditsia sinensis, flowering in May and June, the pod was three or four inches thick, fleshy, with several spots as big as fingers, not round, and white kernels inside, which were edible. Picking pods in October, boiling and mashing, white flour fragrant pills, bathing the body, removing dirt and moistening, is better than Gleditsia sinensis. In addition to natural Gleditsia sinensis, plants such as Sapindus mukoraiensis have also spread among the people and become good cleaners.
In the west, it may be traced back to an ancient Greek island called Lesbos 4,000 years ago. The local people sacrificed animals to heaven. Because wood is used when burning animals, the mixture of wood ashes and animal fat will produce a soapy yellow substance. The heavy rain washed these things into the river where local women often washed their clothes. They found that their clothes were cleaner. Although traces of using similar soap can be found from ancient Greece, a poetess named aponification recorded these stories in history. In memory of her, people later called this process saponification, and the chemical name was soap-based production. In addition, in 3000 BC, Mesopotamia found that the alkaline substances in the ashes after burning plants had detergency after mixing with oil, which was also one of the sources of soap.
However, Gauls should be the first to try to make soap and succeed. At that time, they called soap "sapo". At that time, soap was an ointment-like substance containing animal fat and plant ashes. When the method of making Sapo was gradually introduced to the Mediterranean, Arabs improved Sapo into hard soap made of olive oil and soda. At this time, soap began to be produced in large quantities. Soap was really widely used in18th century and19th century.
At the end of 18, after the advent of the industrial revolution industry, a large amount of cheap sodium carbonate was obtained, which promoted the new development of the soap industry. But in the middle of the 20th century, the development of synthetic chemistry and petrochemical industry provided cheap chemical raw materials for detergents, promoted the rise of synthetic detergents and greatly changed the development of soap industry. However, handmade soap, which has been neglected in recent years, has risen again. Because of its natural and unique properties, handmade soap is easily biodegradable and decomposed by microorganisms in the process of sewage treatment, so it will not cause pollution problems in rivers, lakes and waterways.