Refractive Index: All You Need to Know

Refractometry Index Theory, Measurement, Uses, FAQs and More

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Glossary

  • Wavelength: The length of a single wave measured from one wave peak to the next.
  • Vacuum: An enclosed space from which matter and air has been partially removed.
  • Frequency: The number of waves produced each second. The unit of frequency is hertz (Hz).
  • Refractive index: An index that describes how fast a light beam travels through a particular medium in relation to the speed at which is passes through a second medium. The relationship is described by the formula n = c/v, where c is the speed of light in a vacuum and v is the phase velocity of light in the sample medium.
  • Angle of incidence: Angle between the normal and the incident ray.
  • Angle of reflection: The angle between the reflected ray / wave and an imaginary line drawn at 90 degrees to the reflecting surface.
  • Incident light: Light ray moving towards a surface or boundary.
  • Reflected light: Light ray leaving a surface or boundary.
  • Refraction: Process by which a wave changes speed and sometimes direction upon entering a more dense or less dense medium, e.g. a light ray changing direction when refracted by a lens.
     

Typical Substances, Reference Sample/Materials, and Approximate Refractive Indices

Substance

Refractive Index

Vacuum

1.0000

Air

1.0002

Liquids at 20 °C, wave length of 589.3 nm

1-propanol

1.3848

2,4-Dichlorotoluene

1.5463

Acetone

1.3588

Aloe extract

1.334

Beer

1.346

Bromonaphtalene

1.6578

Butter

1.450

Coconut oil

1.440

Coffee

1.345

Cow milk

1.359

Dodecane

1.4218

Ethanol

1.3338

Glycerol

1.477

Honey

1.520

Ketchup

1.385

Natural rubber

1.540

Peanut oil

1.469

Plain youghurt

1.345

Propylene glycol

1.432

Sodium Chloride

1.334

Sodium hydroxide

1.333

Soy milk

1.350

Sulfuric acid

1.335

Sunflower oil

1.474

Virgin olive oil

1.469

Water (deionized)

1.333

Solids at room temperature

Diamond

2.417

Glass

1.517

 

Applications

Refractive Index Applications – Free PDFs

Related Products

FAQ

What does high refractive index mean?

A high refractive index means that a light beam traveling through media moves slowly and the beam is more deviated (see image below). In practice, the more concentrated a binary substance is, the higher its refractive index.
 

How to avoid solid particle interference when measuring refractive index?

Paste-like samples, e.g. tomato puree, may produce air pockets between the prism and the sample. To ensure that the sample is in full contact with the prism:

  • Stir well before taking a sample. Make sure no air bubbles are introduced during stirring.
  • If it is not possible to completely homogenize the sample, the measurement should be repeated several times and the mean value of the individual measurements calculated.
  • Pour the sample onto the prism and wait a few seconds until dispersed solids sediment on the prism. Use the exact same waiting time for all measurements.
     

How do impurities affect refractive index?

In refractometry there are 2 scenarios of how impurities can affect the measurement:

  1. For liquid impurities that have a higher refractive index than your liquid sample: the velocity of light in the medium will decrease, and therefore increase the refractive index value.
  2. For liquid impurities with a lower refractive index than your liquid sample: the velocity of light in the medium will increase, and therefore decrease the refractive index value.
     

Want to learn how to handle sample with impurities? Download this guide

 

How do solid particles affect refractive index?

If your liquid sample contains solid suspenses, it is recommended to pour the sample into the refractometer sample stage and wait for a fixed period of time (e.g. 10 sec), before starting the measurement.
 

Is it possible to measure the refractive index of solid samples?

Yes, black, dark and colored samples can be measured within seconds using METTLER TOLEDO refractometers. If you measure the refractive index of foils, granulates or gums, we recommend the use of a foil stamp.
 

Why can refractive index measurement be used to identify a sample?

Refractive index can easily be used to identify a pure sample because each element has a unique refractive index. After a measurement, the index of refraction of the sample in question can be looked up to see what it corresponds to. In addition, when using an automatic refractometer refractive index can be automatically converted into other scales (e.g. Brix, %weight/weight, % volume/volume, and more).