Why is foam white no matter the color of the soap?



Why is foam white no matter the color of the soap?

In: Physics

It isn’t. But, the foam may be *lighter* – a lot lighter, even – than the soap because it’s diluted with water.

Because colour comes from how light is reflected off surfaces. Any time there’s a change in the medium the light is travelling through, such as the change between air and water, some of the light bounces off, some of the light changes angle of travel but keeps travelling in roughly the same direction, and some of the light is absorbed by the molecules of the new medium. Also, light has a property called wavelength, which is measured numerically. This wavelength property affects how the light is reflected, refracted and absorbed: For any given medium change, some wavelengths will bounce off, some will change direction, and some will be absorbed, and each of those wavelengths will do that to a certain degree. The “colour” of light is the result of our brains interpreting the wavelengths it is made up of. Light that contains lots of different wavelengths looks white, while light that only contains wavelengths of certain bands will look coloured. Lots of short wavelengths with no long wavelengths looks blue, lots of long wavelengths with no short ones looks red, and middle wavelengths with few short or long ones looks green. Intermediate combinations of wavelengths look the colours between these. For example, some long, some mid, no short will look yellow or orange. Also, purple is a trick of the brain, and it happens when the eyes detect both long and short wavelengths, but no middle wavelengths.

So, let’s imagine you have a bar of soap. This bar of soap is a kind of middle blue colour. It’s not super dark, but it’s not super pale either. It is very blue though, no purple or cyan tinges. This happens because when you shine a white light on the bar of soap, the middle and long wavelengths (the green and red ones) get absorbed by the soap, while the short wavelengths (the blue ones) get reflected. We see only the reflected wavelengths. About 50% of the short wavelengths are reflected to create that middle-strength blue. Hopefully that should make a bit of sense. Now, let’s imagine you have a liquid soap, that’s the same blue colour, but also transparent. This is doing much the same thing when it comes to light, but instead of being absorbed, some of the wavelengths pass through the soap, bounce off the sink surface beneath it, travel back up through the soap and then travel towards your eyes. You still get a blue tinge, because a lot of green and red light is still being absorbed by the soap, but enough is being let through that it also lets you see the stuff beneath the soap.

Now, foam is kind of cool. Foam is actually a bunch of absolutely tiny bubbles, and a bubble is a thin layer of water with a hollow interior, and both the water layer and the air interior are transparent – they let some light through instead of absorbing or reflecting it. Light that passes through a bubble will hit a lot of surface changes. There’s the surface change from the outside air into the water layer, then from the water layer into the inside air, then from the inside air into the water layer again, then from the water layer back into the outside air on the other side. That’s four medium changes, and remember that each time light changes medium, a portion of it will bounce off. If say 10% of light bounces off each time light moves between air and water, then each time, only 90% of light passes through. After 4 transitions, a total of 35% of all the original light has bounced off a surface and back towards the light source. Get a bunch of bubbles, like foam is, and you’ve got hundreds of thousands of opportunities for light to bounce off. Almost no light is getting through, even though the air and water are transparent.

The role soap plays in bubble formation is that it acts kind of like layers holding the water in a bubble shape. It works similarly to how oil behaves in water, forming its own balls of oil instead of mixing with the water. That happens because the oil molecules are something called hydrophobic – they don’t like water and will try to hide from it inside a big ball of oil. Soap molecules are cool, in that one end of them hates air, and one end of them hates water, so when presented with both air and water, the soap molecules spontaneously organise themselves into something called a bilayer: Two rows of soap molecules that trap a layer of water between them, with their water-hating ends pointing out to hide from the water and their air-hating ends pointing in to hide from the air. [Here](https://external-content.duckduckgo.com/iu/?u=http%3A%2F%2Fwww.nanooze.org%2Ffiles%2F2013%2F11%2FdroppedImage-22ss07y.jpg&f=1&nofb=1) is a kind of OK diagram of that. The molecules responsible for the blue colouration of the soap aren’t involved in this process though, and water reflects light of all colours, so the special property of only reflecting blue light ends up lost when the soap molecules are mixed with water and air to make foam. Those blue colour molecules, which aren’t the soap molecules but instead a dye molecule put in to make the soap look blue, are just tinging the foam a pale blue colour, as the water bubbles are doing loads of white reflection, but scattered blue dye molecules do still absorb some of the red and green.

Because many colors of light blended together look white and the bubbles are bouncing those colors in all different directions.

Light bounces off of objects until one colour sticks. Foam is made out of lots and lots of little objects, so the light bounces in all sorts of different directions. A colour cannot stick, and so it looks white.

“white” is actually just all colors (additive) of the visible spectrum being detected at once, which occurs in the soap molecules as the light that is hitting or passing through those molecules is “bent” or angled in such ways that allow for absorption/transmission/reemission of different wavelengths, resulting in our eyes detecting “white”

Imagine looking at a bubble, and how its got all these different kinds of colours on its surface, the reason for this is different thicknesses in the bubble.

Now imagine soapy foam, it’s a multicolored bubble, but there’s thousands/millions of them… your eyeballs aren’t equipped to see such detail, so it gets “averaged out”, and with the combined effects of the light being scattered through all the bubbles, we interpret it as white… which is a good approximation for “all the colours” in our brain

The reason is because foam is made out of tiny little bubbles. The bubbles refract the light in a way that makes it look white.

A great example of this effect is in white candy. White candy often does not use food dye, but is instead clear candy that is folded a bunch of times to trap little air bubbles. [Here’s a video of it in action](https://youtu.be/qZohC_UUdE4?t=167)

Having worked in kitchens, soup foam is not always translucent. You just haven’t seen enough.

While it seems a very good answer has been given, here is a great ELI5 video if you want some visuals with it (first time posting links in Reddit, hope it works)