answer:
Everyone must have blown bubbles, and there must be careful friends who have found similar problems, why do everyone blow out the bubble color is different? Some don't even have colors
In fact, from the perspective of optics, the reason for this problem is the interference of light, if you go into detail, it is a thin film interference phenomenon. Thin film interference: a beam of light waves shines on a thin film, due to the different refractive index of the film and the outside world, the light waves will be reflected by the upper and lower interfaces of the film, at this time the reflected light will be interfered with because of the phase to form a new light wave, this phenomenon is called thin film interference.
When a bubble is born to burst, the thickness of the bubble has actually been changing, and the interference situation when the thickness of the film changes is also constantly changing, in fact, we can infer the thickness of the bubble from the color change of the bubble [1]!
As we can see from the above figure, when the thickness of the bubble keeps changing, the color we see is also constantly changing, because the thickness of the bubble itself is uneven, so the bubble color we see is colorful. However, when the thickness of the bubble is less than 100 nm, the colorful color of the bubble becomes colorless (white) because the thickness of the film is too small to affect the color of the reflected light, which is what the subject calls the colorless bubble.
Resources:
Soapy water is obviously transparent, so why does the bubble become colored?
by justiu
Q.E.D.
Q4
by Anonymous
In general, it is approximate.
We can use the law of refraction to quantitatively calculate this problem. Consider a beam of parallel light incident on a flat convex lens, the light path diagram is shown below
The directional light first passes through a plane perpendicular to the light, at which point the direction of the light does not change. Refraction then occurs again on the sphere, and the angle of incidence and angle of refraction satisfy the law of refraction
Depending on the geometric relationship, it is possible to write the distance between two points of the OA
The distance and angle of a parallel ray to the main optical axis is
According to the above three equations, it can be calculated that the distance from point A to the center of the sphere where the light converges from the main optical axis is different from the distance to the center of the sphere
It can be seen that the change is not a constant constant. The drawing looks like this (take the refractive index)
It is clear from the figure that the incident directional light does not converge on the same point.
However, in most simple optical systems, we use the "Pong axis approximation", that is, considering the light that is very close to the main optical axis, if only this part of the light is considered, it can be approximated that these rays can converge on the same point, which is the "focus".
However, it is still possible to design a more complex shape of the lens surface so that the light can accurately converge on the same point, and interested students can calculate the shape of the surface by themselves ~ (the answer is ellipsoidal)
by Paarthurnax