Re: Does changing the concentration of a liquid change its refractive index?

Hi Karen,

You’ve asked a very good question that a lot of people (physicists 
included), don’t always consider when they look up an index of 
refraction.  The refractive index depends on a lot of things, and isn’t 
always the same number in all situations.

The short answer is: Yes, changing the concentration of salt will change 
the index of refraction enough for you to measure.

The detailed answer is: Increasing the concentration of salt in water 
will affect the density and atomic interaction of the water and will 
increase the index of refraction.  The change in refractive index will be 
on the order of hundredths, resulting in an change in refractive angle of 
over a degree.

The even more detailed answer is: Temperature of the water and the 
wavelength of the light used to measure the angle of refraction will also 
affect the index of refraction.  So any value given for the index of 
refraction of a material (especially a liquid) should specify exactly 
what wavelength was used, what temperature the material was, and what 
concentration of stuff was in the solution.  The bad news is that there’s 
not a single equation that you can use to predict or calculate refractive 
indices.  The good news is that when chemists and physicists need to 
figure out an index of refraction for a solution or a material, they do 
exactly what you’re doing right now.  They come up with an experiment and 
test it out.  

You should be able to measure a difference for varying concentrations of 
salt water.  One student did a very 
similar experiment (PDF format) with sugar water, and your results 
should be in the same ballpark.  Her values correspond to about a degree 
and a half difference between 0% and 20% concentrations.  If you use a 
laser pointer, a fishtank, and some trigonometry, you should be able to 
measure this with no problem.  If you decide to use other liquids, 
vegetable oil, air, and water are immiscible (they don’t mix) and have 
different indices of refraction.  So these are commonly used to 
demonstrate a double refraction.
 
I hope this helps.

Jeff Yap
Mad Scientist

Some background info:  An index of refraction is the ratio of the speed 
of light in a material to the speed of light in a vacuum.  Because light 
will slow down in every material, the wavelength gets squished closer and 
the beam bends.  Imagine a marching band that goes from the pavement into 
heavy mud.  If they’re going straight into the mud, the ranks (rows.  
Files are columns.) get closer together, but they’re still travelling in 
a straight line.  However, if they enter the mud at an angle, the person 
on the corner is going to hit the mud and slow down before the person in 
the middle, who is going to slow down before the person on the opposite 
corner.  When this happens, the direction the marching band is going is 
going to change.  The higher the refractive index, the more a material 
will bend a ray of light (deeper mud for the marching band).  Here’s a 
Java applet that shows it pretty well.  There’s also something known 
as total internal reflection, which is where rays are at such a steep 
angle, they can’t get out of the mud, and just bounce back from the 
edge.  This is how fiber optics work.  There’s another cool demo where 
you can make Pyrex disappear in Wesson oil because they have the same 
index of refraction.  (Lots of other cool videos on this page too.)