A measure of how much light bends passing through a substance, used to identify liquids.
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The refractive index n of a substance is the ratio of the speed of light in vacuum to its speed in the material, n = c/v, and it describes how much light bends when it crosses an interface. Refractive index depends on wavelength, which is why measurements use the sodium D line at 589 nm, and it also changes with temperature, so refractometers are thermostatically controlled. Because a pure liquid has a characteristic refractive index, measuring it gives a quick test of purity. The refractive index of a solution changes with concentration, so refractometers are used for sugar content in juices, antifreeze strength in coolant and oil quality. The measurement requires only a few drops of sample.
Key ideas: Snell's law: n1 sin theta1 = n2 sin theta2. Refractive index falls as temperature rises by about 0.0004 per degree for organic liquids. Quote as n 20 D, meaning at 20 C using the sodium D line. Abbe refractometers use the critical angle. Specific refraction links n and density and is nearly constant for a compound, and water at 20 C is 1.3330.
Common mistakes and connections: Students forget that n is greater than or equal to 1 and has no units, or compare values taken at different temperatures. Sample evaporation changes readings, and contamination of the prism surface leads to errors. Refractive index alone does not prove identity, as different compounds may coincide. Related to speed of light, total internal reflection and purity checks.
Refractive index at 20 C and the sodium D line is a characteristic constant of a pure liquid, and it shifts with solution concentration. The interactive lesson shows this as a live animation or chart.
Water has a refractive index of 1.333 and ethanol 1.361 at 20 C.
A measure of how much light bends passing through a substance, used to identify liquids.
Refractive Index is a intermediate topic in Analytical, Lab & Spectroscopy. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Analytical, Lab & Spectroscopy first: Chemiluminescence, Atomic Emission Spectroscopy, Polarimetry.
The next topics in Analytical, Lab & Spectroscopy are Melting Point Determination and Boiling Point Determination.
Yes. The interactive lesson includes a self-check question on Refractive Index, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Refractive Index is topic 77 of 99 in Analytical, Lab & Spectroscopy.