# UV-Vis spectroscopy: Changes in refractive index

• Ella98
In summary, the beam hits the surface of the liquid at an angle and the refractive index of the liquid changes how much light passes through the cuvette. Changes in refractive index will affect the reflectivity of the spectrometer.
Ella98
I wanted to know how do you control for changes in refractive index when you get the UV-Vis spectrum of an aqueous solution in a double beam spectrometer? Is that something the equipment does automatically? Or something I have to do? Also, how do these changes in refractive index occur? Thanks for any help you can provide.

How does the beam hit the surface of the liquid? At what angle?

Ella98
Borek said:
How does the beam hit the surface of the liquid? At what angle?
The beam is perpendicular to the surface of the cuvette holding the liquid. Thanks!

Ella98 said:
The beam is perpendicular to the surface of the cuvette holding the liquid. Thanks!
It is not going to refract if the incident beam is perpendicular to the surface. Changes in refractive index will change the reflection though.
$$R=\left |\frac{n_{1}-n_{2}}{n_{1}+n_{2}} \right |^{2}$$

Last edited:
Ella98
HAYAO said:
It is not going to refract if the incident beam is perpendicular to the surface. Changes in refractive index will change the reflection though.
$$R=\left |\frac{n_{1}-n_{2}}{n_{1}+n_{2}} \right |^{2}$$
Will changes in reflection affect my absorbance measurements? I just want to know if the changes in refractive index will affect my absorbance measurements. Thanks, Hayao.

It might be nice if you could attempt a reflectivity calculation on your own of the case where there is only water in the cuvette, and then think about how much that changes when there is a sample dissolved in it. Don't ask for "do it for me". Try your own attempt and we'll guide you.To start you off, look up refractive index for air, water, and cuvette (most likely made of quartz). And then put the numbers in the Frensel reflectivity equation above for both intersection, namely air-quartz and quartz-water. At this point, you will have to make an assumption based on these numbers whether you can neglect multiple reflection (reflection of a reflection of a reflection of a...) or not. Then calculate how much of the initial light passes through the cuvette with water. (Note: we have four intersection in total, i.e. two quartz walls.)

If you have done that, then make a reasonable estimate of how much the refractive index of the water changes when you have something dissolved in it. Contemplate how much that would affect the reflectivity.

HAYAO said:
It might be nice if you could attempt a reflectivity calculation on your own of the case where there is only water in the cuvette, and then think about how much that changes when there is a sample dissolved in it. Don't ask for "do it for me". Try your own attempt and we'll guide you.To start you off, look up refractive index for air, water, and cuvette (most likely made of quartz). And then put the numbers in the Frensel reflectivity equation above for both intersection, namely air-quartz and quartz-water. At this point, you will have to make an assumption based on these numbers whether you can neglect multiple reflection (reflection of a reflection of a reflection of a...) or not. Then calculate how much of the initial light passes through the cuvette with water. (Note: we have four intersection in total, i.e. two quartz walls.)

If you have done that, then make a reasonable estimate of how much the refractive index of the water changes when you have something dissolved in it. Contemplate how much that would affect the reflectivity.

I'm sorry I didn't mean with my reply to imply I wanted you to do any calculations for me. I just thought what I asked was a yes or no question. I didn't know calculations will be required to answer it. Thanks for the help! I really appreciate it.

Ella98 said:
I'm sorry I didn't mean with my reply to imply I wanted you to do any calculations for me. I just thought what I asked was a yes or no question. I didn't know calculations will be required to answer it. Thanks for the help! I really appreciate it.
And that's not what I meant, neither.

You can't understand something with simple yes or no. You understand it by really doing the calculations on your own, even if you intuitively understand why. I wanted you to really understand it. What if you had to teach someone how to use UV-Vis absorption spectrometer and you were asked the same question? Yes or no isn't always the answer that they are looking for.The answer is it's negligibly small enough, btw.

## 1. What is UV-Vis spectroscopy?

UV-Vis spectroscopy is a technique used to measure the absorption and transmission of light by a sample in the ultraviolet and visible regions of the electromagnetic spectrum. It is commonly used to analyze the chemical composition and concentration of a substance.

## 2. How does UV-Vis spectroscopy measure changes in refractive index?

UV-Vis spectroscopy measures changes in refractive index by analyzing the absorption of light by a sample. When light passes through a sample, it may be absorbed by the molecules present. The degree of absorption is related to the refractive index of the sample, which can then be calculated using mathematical equations.

## 3. What type of samples can be analyzed using UV-Vis spectroscopy?

UV-Vis spectroscopy can be used to analyze a wide range of samples, including liquids, solids, and gases. It is commonly used in the fields of chemistry, biochemistry, and environmental science to study the properties of various substances.

## 4. What are some common applications of UV-Vis spectroscopy?

UV-Vis spectroscopy has many applications, including determining the concentration of a substance, identifying unknown compounds, and studying chemical reactions. It is also commonly used in quality control and research and development in industries such as pharmaceuticals, food and beverage, and cosmetics.

## 5. How does UV-Vis spectroscopy compare to other analytical techniques?

UV-Vis spectroscopy is a relatively simple and fast technique compared to other analytical methods. It also has a wide range of applications and can provide quantitative and qualitative information about a sample. However, it may not be suitable for samples with high levels of turbidity or fluorescence, and it may not provide information about the structure of a compound like other spectroscopic techniques such as infrared spectroscopy.

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