Muhammad Hamza El-Saba

First of all thank you very much for your answer. I would actually like to know how nanoparticles cuantifiers work. If they work with light, how can they measure nanoparticles? Taking into account light wavelength is 400 - 750 nm.

Hi Borja Abascal , for dynamic light scattering, the measurements works by looking at the effect that Brownian motion has on the light: from the fluctuations in the intensity we get information about the diffusion of the particles. That diffusion coefficient is then mathematically turned into a size. In a way, the statistics of the scattered light of an ensemble of nanoparticles are used to infer the average size of the ensemble.

(It is actually possible to also get some information about size distributions from DLS, but I tried to simplify for you. https://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/ )

Dear Borja Abascal , of course wavelength is important when you are using waves to measure something, but it is just one of several parameters that you can use to measure the size of something. Following your reasoning it would be impossible to measure the thickness of a paper sheet with a rule where the small division would be 1mm, and in fact this is possible.... you only need for instance, to have a thick stack of these paper sheets, measure its thickness and divide the value by the number of sheets... The bigger the number of sheets, the best the resolution.

So the key is actually how you use your ruler.

When you are dealing with scattering by nanoparticles (particles with size well below the wavelength of the light used) the scattering phenomenon is described by Rayleigh scattering:

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

Mie theory describes the scattering caused particles with sizes close or larger than the light wavelength.

https://en.wikipedia.org/wiki/Mie_scattering#:~:text=Mie%20scattering%20(sometimes%20referred%20to,incident%20ray%20may%20be%20present.

John Tyndall observed the scattered of visible light of colloids and of nanoscopic particles. The Tyndall effect is well known to all of us, for example when a ray of Sun light becomes visible when dust is in suspension. Many of these tiny and bright particles are smaller than the wavelength of light, and because light interacts with them causing scattering we can see them. Actually they behave like tiny mirrors. This is also what you see when a visible laser beam hit some dust.

If your experiments with visible light scattering are related to Dynamic Light Scattering (DLS), the measurement principle is a bit more elaborated, as clearly stated by @Alan F. Rawle in this RG post:

https://www.researchgate.net/post/How-to-use-dynamic-light-scattering-DLS-To-obtain-the-size-of-nanorods

Hope it helps

Ulf Nobbmann Thank you very much for your answer

Dear Borja, you ask a great question. We all know the sky is blue because of the Rayleigh scattering of the light by the atmosphere. The air molecules (tenths of nm in diameter) are much shorter than visible light (400 -800 nm). So what is scattering the light? It turns out that density fluctuations are scattering the light. The brownian motion causes groups of molecules to congregate together and other places to have fewer gas molecules. These density fluctuations depend on the size of the molecules and other physical conditions (like temperature and pressure). Thus, light scattering is really measuring density fluctuations in the nanoparticles. So, you are measuring "clusters" of particles and clusters that are much larger than the size of one gas molecule. From the dynamics (changing) of these density fluctuations, we can infer the individual particle sizes.

Now, my question for you....we all know that Rayleigh scattering depends on 1 over wavelength to the fourth power. Thus, shorter wavelengths are scattered more. If this is true, why is the sky blue? It should be violet (a shorter wavelength).

Lou Reinisch thank you very much for your answer. I also think we should see the sky violet. As far as I know, it has something to do with our optical receptors, they process blue waves in a better way, so I suppose the sky is indeed violet. If you have more information about this issue let me know, you piqued my curiosity.

Once again thank you!

Excellent....yes the sky appears blue because of the spectral response of our eyes. The sky is, in fact, violet. That was a major problem with some of the first digital cameras. The receptors in those cameras were fairly flat across the visible spectrum and the sky appeared violet instead of blue. We must also remember that not only is the illumination important, but the detector response is equally as important. Continue to question all that you are told. That is necessary to really understand. I wish you the best.