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Bubbles (water) are usually translucent, and transparent too.

In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one where the dimensions investigated are much larger than the wavelength of the photons in question), the photons can be said to follow Snell's Law. Translucency (also called translucence or translucidity) allows light to pass through, but does not necessarily (again, on the macroscopic scale) follow Snell's law; the photons can be scattered at either of the two interfaces, or internally, where there is a change in index of refraction. In other words, a translucent material is made up of components with different indices of refraction. A transparent material is made up of components with a uniform index of refraction.[1] Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color.

https://en.wikipedia.org/wiki/Transparency_and_translucency

But foam is always white, though it is made up of transparent constituents (waterbubbles).

I am not asking why foam is always white. I am asking how a transparent material, like water in bubbles, in big amounts will produce white light.

Now this might seem obvious, because transparent materials are reflecting off all light that is shone on them.

But, white (natural) light is a combination of certain wavelengths.

White objects fully reflect and scatter all the visible wavelengths of light. While there is no single, unique specification of "white light", there is indeed a unique specification of "white object", or, more specifically, "white surface". A perfectly white surface diffusely reflects (scatters) all visible light that strikes it, without absorbing any, irrespective of the light's wavelength or spectral distribution.[29][30] Since it does not absorb any of the incident light, white is the lightest possible color. If the reflection is not diffuse but rather specular, this describes a mirror rather than a white surface.[31][29]

https://en.wikipedia.org/wiki/White#White_light

So basically, white objects, like foam, should reflect all visible wavelengths of light, though they are comprised of transparent objects, bubbles, that are refracting all visible wavelengths.

And, wet objects are usually darker, not brighter.

Why are so many different types of objects white, yet appear gray when they are wet?

So this cannot explain it either.

Question:

  1. So how does adding bubbles together change from total refraction (transparent) to total reflection (white) of all visible wavelengths?
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  • $\begingroup$ You said "So basically, white objects, like foam, should reflect all visible wavelengths of light" and it does but the foam and the reflections are not uniform so the nicely dispersed and rflected light is all mixed up again to create white. $\endgroup$ – Bill Alsept Mar 11 at 22:32
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a beam of light that strikes a bubble in a transparent liquid experiences a sudden change in the index of refraction. This means that depending on the exact angle of incidence of the light ray on the (curved) surface of the bubble, the light ray will get split- part of it will bounce off the bubble, part will enter the bubble, and the part that enters the bubble will be split again, with part of it being reflected back into the bubble and part will go through the bubble wall and leave the bubble.

In addition, the curved surface of the bubble will act as a lens and bend the reflected and transmitted beams in a variety of ways, again depending on the angle of incidence.

Any beam of light entering a large cloud of bubbles will be reflected, refracted and transmitted many times as it bounces from bubble to bubble, meaning that each bubble will be illuminated by light rays coming at it from all directions- and the whole bubble cloud will appear white to your eyes.

The net result for a large cloud of small bubbles illuminated by the sun will be a sparkly white, foamy appearance. the smaller the bubbles, the more "milky" the appearance; the larger the bubbles, the more "sparkly".

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  • $\begingroup$ The best example of this is a white cloud. $\endgroup$ – Bill Alsept Mar 11 at 23:36
  • $\begingroup$ yep. inside-out air bubbles in water! $\endgroup$ – niels nielsen Mar 12 at 2:00
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So how does adding bubbles together change from total refraction (transparent) to total reflection (white) of all visible wavelengths?

No interface will give you total refraction. Some light will be reflected. For a straight-on clean sheet of glass, you might get a large transmission fraction, maybe above 99% in some cases. For most appearances, the object is perceived as transparent.

But there's still some reflection. For a single sheet of glass (or a single bubble), that might not matter much. But increase the number of interfaces so you have hundreds or thousands of interactions, and now the chance of transmitting through every one consecutively drops to near zero. The light coming through the material is no longer focusable, but blended due to the multiple, random reflections. Instead of seeing objects behind, you see mostly an average of the incident light (which is usually perceived to be white).

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Incoming light on each bubble surface is most refracted, but a small part is reflected. And when light is entering the bubble, there is a reflection at the interface water-outside and another at the interface water-inside.

Depending on the local thickness, and the angle to our eyes, some wave length will have maximum construtive interference in the reflection. It is the same reason for colours on the surface of water contaminated with oil. The oil floats and form a very thin surface, that generates colours by constructive interference.

The difference of foam, is that colours of random wave lengths come to us from thousand or millions of small surfaces, and that combination results in a white light.

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