Difference in refractive index between tap water and salt water
L. Snel & N. Chekrouni
Het 4e Gymnasium, Amsterdam
Summary
When light travels from air to a liquid the direction of light changes. This phenomenon is called
refraction. Each medium has its own refractive index. The purpose of the experiment was to find out
if the refractive index of salt water differs to the refractive index of tap water. Four different solution
had been taken for the experiment. After measuring the refractive indices, the refractive index of salt
water turned out to be bigger than the refractive index of tap water.
Introduction
Refraction is the bending of a (light)wave when it
passes from a fast medium to a slower one. The
amount of bending depends on the index of refraction,
which differs for each sort of
medium. We know that there
is a constant ratio between
the sine of the angle of
refraction θ1 and the sine of
the angle of refraction θ2
when a light beams from
vacuum
and
enters
a
transparent
medium,
according to Snell's Law.
This constant ratio is also called
the refractive index n:
This raises the question: Does tap water differs in
refractive index from salt water?
To make salt water, sodium chloride has been
used. Sodium chloride dissolves easily in water and
separates this into Na+ and Cl- ions according to
the following equation:
NaCl (aq)  Na+ (aq) + Cl- (aq)
Refraction
When the angle of refraction θ2 is bigger than 90°, total
reflection occurs. Because an angle bigger than 90 NaCl is separated into Na+ and Cldegrees would prevent us to determine the angle of
refraction θ2, there is a limit g to angle of refraction θ1. Research showed that the maximum amount of
This critical angle is determined by
salt that dissolves in one liter of tap water is 359
grams. In order to make a solution in which the
maximum amount of NaCl can dissolve you only
need 80% of that amount.
where n1 and n2 are the respective refractive indices.
Experimental procedure and data
Each and every medium has its own refractive index.
When light travels from air to water, the water will analysis
refract differently then when light refracts from air to Four identical measuring cylinders had been taken.
The first measuring cylinder was filled with 1 liter
another substance.
of tap water. The second measuring cylinder was
also filled with 1 liter of tap water and 287,2 grams of salt
was added, which is 80% of the maximum amount of salt
that can dissolve in water.
The third measuring cylinder was filled with 0,5 liter of
the second solution, and then added 0,5 liter of tap
water, so that the third solution was two times more
diluted than the second solution. In the fourth measuring
cylinder 0,5 liter of our third solution had been added
and then we added 0,5 liter of tap water again, so that
the fourth solution was a two times more diluted than
our third solution.
The supplies that were used for this experiment were:
- A bowl with a magnetic strip
- A magnetic protractor
- A laser light attached to a magnet
The protractor was attached to a magnetic board and the
bowl was filled (also attached to the board) with the first
solution, with such precision that the solution surface
was exactly on the x-axis of the protractor. Then the laser
light also was attached to the magnetic board on the
side of the negative x-axis and a positive y-axis. The angle
of refraction θ1 was variable but in such manner that no
reflection occurred (smaller than the critical angle of
48,6°). Then the angle in the solution was been observed,
by looking at it perpendicularly. In this way angle of
refraction θ2 was observed. These observations were
repeated several times so that the results would be as
accurate as possible. After finishing the observations with
the first liquid (tap water) the experiment continued the
same way as written above but with a difference in
liquid/solution.
Results
Each experiment for each solution has be repeated three
times, each time with a different angle of refraction n1.
Water/sol.1
Sol.2
Sol.3
Sol.4
st
1
Sin 30°/ sin
Sin 30°/
Sin
Sin 30°/
22° = 1,33
sin 20°=
30°/sin
sin 23°
1,46
22 °=
= 1,28
1,33
2nd
Sin 35° / sin
25° = 1,35
Sin 35°/
sin 25° =
1,36
Sin 35° /
sin 26°
= 1,31
Sin 35°/
sin 26°
= 1,31
3rd
Sin 40°/ sin
29° = 1,33
Sin 40°/
sin 29 °=
1,32
Sin 40° /
sin 30°
= 1,29
Sin 40°/
sin 30°
= 1,29
avera
ge
1,337
1,381
1,309
1,291
Table 1: refractive indices of four solutions
Table 1 shows the results of the experiments. The
experiment is repeated three times so at the end
the average of the three experiments could be
taken. This gives better and more accurate results.
Keep in mind that solution 1 tap water is, and that
solution two times stronger is than solution 3 and
that solution 4 contains the least NaCl.
Conclusion / discussion
The conclusion that can be drawn from the results
is that salt water definitely has a higher refractive
index then tap water. This occurs because of the
separation of NaCl into Na+ and Cl- ions. Those ions
are diffused all over in the water, therefore it
becomes more difficult for the water to bend the
incoming light beam like it does when it is only tap
water.
Also it is noteworthy that the difference in
refractive index between the less dissolved
solutions is lower than expected. This is due to
the low amount of NaCl that is dissolved in water
and doesn’t show such a powerful effect as the
ones with a higher concentration of salt. It is
strange that the third and fourth solution have a
lower refractive index than water.
Perhaps other salts could have been used, in order
to achieve some more interesting results, with
perhaps also differences in refractive index. Also
the accuracy of the experiment could have been
more precise, as noticeable from the
abnormalities in the results. This is due to the
difficulty of keeping the bowl straight on x-axis of
the protractor when it is filled with water or
another solutions and becomes more heavy.
Bibliography
1. BINAS table 18B
2. http://nl.wikipedia.org/wiki/Brekingsind
ex
3. http://en.wikipedia.org/wiki/Refractive_i
ndex
4. Iru, P., Luib, A., & Nelem, D. (2010)
Journal of anorgano chemistry. The effect
of NaCl (s) on ice