# Suspended particles and the index of refraction

• PenderJ
In summary, the presence of suspended sub-micron sized particles can affect the index of refraction of a liquid, particularly for visible light which falls into the realm of Mie Scattering. Removing these particles can result in a lower reading on a refractometer due to the elimination of scattering. For particles smaller than lambda/pi, they are inductive and their radiation lags behind the incident E and B waves, resulting in a slower radiation and an increased index of refraction for the material.

#### PenderJ

Do suspended sub-micron sized particles influence the index of refraction of a liquid?

Drakkith
I guess it isn't an intermediate question. How do you edit it to change the level? Is there a level that corresponds to it being too hard for this forum?

For visible light this is a scattering problem of some complexity: ##1\mu m=10^4Angstrom##so the particles are slightly larger than the wavelength so this is the realm of Mie Scattering and not just the index of refraction which is usually applied to more homogeneous materials. Certainly the physics is interesting .

vanhees71
So when I filter out these particles and observe a lower reading in a refractometer I'm actually observing a change due to the elimination of scattering. And predicting this quantitatively would be complicated. Is that correct?

When you say sub micron, are we talking about smaller than the light wavelength?

What type of refractometer? The answer to your question is very probably yes. Is the liquid with particles still nearly transparent?

I'm using 0.45 micron filters. The liquid is translucent and a little cloudy prior to filtration and has good clarity afterwards. The liquid is espresso coffee.

I have an el cheapo (\$25) hand held optical refractometer, the kind where you look for the shadow line through the eyepiece. Prior to filtration the line is fuzzy but it is also higher on the scale. I have read about the same phenomenon (higher reading, less precise) being observed with more expensive electronic refractometers.

I was just curious what exactly was going on to change the reading. I get the fuzzy part, being that the liquid is cloudy. But why would it read higher?

This is what I suggest is happening. If the particles are smaller in diameter than lambda/pi then they are too small for resonance. This means they are inductive. The incident E-field causes a circumferential current on the surface of the particle and this current lags the E-field by 90 degrees. The current results in radiation which is 90 degrees delayed relative to the incident E and B waves. So the radiation from the particles is slower than free space. This gives the material an increased Index of Refraction.

tech99 said:
This is what I suggest is happening. If the particles are smaller in diameter than lambda/pi then they are too small for resonance. This means they are inductive. The incident E-field causes a circumferential current on the surface of the particle and this current lags the E-field by 90 degrees. The current results in radiation which is 90 degrees delayed relative to the incident E and B waves. So the radiation from the particles is slower than free space. This gives the material an increased Index of Refraction.

Excuse my ignorance but by lambda/pi do you mean roughly 1/3 the wavelength of light? The particles I'm talking about are larger than 450nm.

## What are suspended particles and how do they affect the index of refraction?

Suspended particles are small particles that are dispersed in a medium, such as air or water. They can be solid, liquid, or gas and can vary in size from nanometers to micrometers. These particles can affect the index of refraction by scattering light, which alters the path and speed of light as it passes through the medium.

## How does the index of refraction change with different types of suspended particles?

The index of refraction is dependent on the physical properties of the suspended particles, such as their size, shape, and composition. Different types of particles can have different refractive indexes, which can affect the overall index of refraction of the medium they are suspended in.

## What is the relationship between the concentration of suspended particles and the index of refraction?

The concentration of suspended particles can have a direct impact on the index of refraction. As the concentration increases, there are more particles present to scatter light, resulting in a higher index of refraction. Conversely, a decrease in concentration can lead to a lower index of refraction.

## How do suspended particles affect the transparency of a medium?

Suspended particles can reduce the transparency of a medium by scattering and absorbing light. When light passes through a medium with suspended particles, the particles can scatter the light in different directions, making it more difficult for light to pass through and decreasing the transparency of the medium.

## Can the index of refraction of a medium with suspended particles be calculated?

Yes, the index of refraction of a medium with suspended particles can be calculated using the concentration and physical properties of the particles. However, the calculation may be complex and may require specialized equipment to accurately measure the index of refraction in the presence of suspended particles.