# UV-Vis spectroscopy: Changes in refractive index

1. Apr 10, 2018

### 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.

2. Apr 10, 2018

### Staff: Mentor

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

3. Apr 10, 2018

### Ella98

The beam is perpendicular to the surface of the cuvette holding the liquid. Thanks!

4. Apr 10, 2018

### HAYAO

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: Apr 10, 2018
5. Apr 11, 2018

### Ella98

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.

6. Apr 11, 2018

### 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.

7. Apr 11, 2018

### Ella98

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.

8. Apr 11, 2018

### HAYAO

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.