minisea
New Member
That was a great write-up by Randy, but I must add that #1 can also be considered a correct answer, as the wavelength in a given material is linearly proportional to the speed of light. The equation c=l*f states that the speed of light in a given material (c) is equal to the frequency (f) multiplied by the wavelength (l). Since the frequency of light doesn't change upon entering a material, the wavelength changes in proportion to the speed of light. So you can define/calculate the refractive index based on wavelengths too.And the answer is:
Which of the following is a refractometer measuring to determine seawater salinity?
1. Changes in the wavelength of light passing through the sample
2. Changes in the speed of light passing through the sample
3. Changes in the intensity of light passing through the sample
4. Changes in the frequency of light passing through the sample
Refractometers and Salinity Measurement - REEFEDITION
Title photo by R2R member Rskillz By Randy Holmes-Farley Salinity is one of the most important parameters measured in reef aquaria. It controls not only the salt balance between an organism and its surrounding environment, but also the levels of a host of ions Read more here...www.reefedition.com
What is the Index of Refraction?
The index of refraction (or refractive index) is the ratio of the speed of light traveling through a vacuum to the speed of light in the material being tested. Most aquarists do not realize that when using a refractometer, they are measuring the speed of light through their aquarium’s water, so having such knowledge might be a good way to impress friends with your technical abilities!
Light travels through most materials more slowly than it does through a vacuum, so their refractive index is higher than 1.00000. The detailed mathematics and physics behind refractive index are actually quite complicated, because it is often a complex number with real and imaginary parts, but a simple version is adequate for all purposes that a reef aquarist would encounter. Some materials slow light traveling through them more than others, and slower light travel leads to a higher refractive index. Table 1 shows some typical refractive index values for comparative purposes.
In solutions of two compounds, such as ethyl alcohol in water, sugar in water or salt in water, the refractive index changes in step with how much of each component is present. Scientists have long known this to be true, and refractometers have a long history of use in brewing, sugar refining, analyzing blood and urine protein and many other industries where a quick measure of refractive index can lead to a good assessment of what is present.
Refractive index generally cannot reveal the identity of compounds in water, but when an aquarist knows roughly what material is there he can determine how much of it is there (within the refractive index’s detection capability). Changes in refractive index are not suitable for determining trace levels of ions (such as the purity of freshwater coming out of an RO/DI (reverse osmosis/deionization) purification system), but it can do a good job when significant amounts of a known material are present.
For example, refractive index cannot determine whether a salt in water is potassium sulfate, sodium chloride, magnesium nitrate or calcium bromide, but if you know which of these you have by some other means (such as the name on a chemical’s bottle), then you can determine how much is present in solution by measuring the refractive index, and then looking it up in a table that relates the refractive index to the concentration of that material.
Refractive Index and Salinity
Aquarists can use the effects that added salts have on the refractive index of a water solution to determine the salinity of reef aquarium water. As the salinity of seawater rises, the amount of salt added rises, so the refractive index rises. Figure 1 plots seawater’s refractive index vs. its salinity. Figure 2 shows a similar plot of seawater’s refractive index vs. specific gravity. These data are also summarized in Table 1. These sets of data demonstrate how aquarists can use refractive index to measure salinity and specific gravity, assuming they have a refractometer that can read in the appropriate refractive index range.
Figure 1. A plot of the relationship between the refractive index and the salinity of seawater.
