Basics of refraction

Refractive index measurement is used to determine the characteristics of liquid and solid samples. For example, measuring the concentration of solutions is done using the refractive index. Identifying this coefficient provides the possibility of controlling the quality of multi-component mixtures and checking the samples in terms of purity. This article provides you with an overview of the topic.

What is refractive index?

Any material that interacts with light has a refractive index (RI symbol n). This parameter is constant and dimensionless and describes how the speed of light travels through a specific medium compared to the speed of light through a reference medium (usually vacuum or air). The lower the optical density of the environment, the faster the speed of light inside the environment and the lower its refractive index.

Examples of the speed of light in different environments

This parameter in a material depends on the wavelength λ of light and the temperature T of the material. If its measurement is not done in standard conditions; Wavelength and temperature are determined by referring to this coefficient. The standard wavelength for measuring refraction is 589 nm. In this situation, the refractive index (n) is often written as nD. Several scales of this parameter have been used. The ratio of the speed of light in vacuum to the ratio of the speed of light in the medium shows the absolute refractive index of the medium. The most common method in refractometry is to use the refractive index compared to air. Since the absolute refractive index of the target medium as well as the refractive index of air depends on the temperature, it is necessary to determine the details.

Common variables in the concept of refractive index

• The ratio of the refractive index of the temperature-dependent medium to that of standard air (standard air; it has a temperature of 20˚C, 1013 hPa and 50% relative humidity)
• The ratio of the refractive index depends on the ambient temperature on the air ratio (air temperature and ambient temperature are the same)

How to detect the critical angle

The easiest way to understand this phenomenon is to visualize the actual position and the observed position of a fish underwater. The refraction of light that leaves the water and reaches the air makes us see the position of the fish closer than it is!

Figure 1) The refraction of light reflected from the fish (from the sun) makes us see the position of the fish closer than it actually is.

Figure 2) The greater the difference in refractive indices, the greater the change in direction.

Figure 3: Refraction of light from fish, in different positions.

In beige fish: part of the light is refracted into the air and another part is reflected back into the water.

In red fish: light is refracted exactly at the boundary surface and does not enter the air.

And in the blue fish: no light rays are refracted into the air; All light rays are reflected into the water (total reflection).

Figure 4: Total reflection

Figure 5: Using the example of a fish in a refractometer:

Consider prism (n1) instead of lake and sample (n2) instead of air.

Law Asnal

Figure 6: According to their angle, incoming light rays are completely reflected (criticality α>α) or part of them is refracted and another part is reflected (criticality α<α).

In this diagram, ray A hits the surface of medium 1 (refractive index = n1) and medium 2 (refractive index = n2 and n2>n1) at an angle. This collision leads to partial reflection (ray ´A) and partial refraction (ray A at angle β1) at the boundary surface.

At a specific radiation angle α2, part of the light beam is reflected and the other part is refracted exactly along the interface of the two media. This type of total reflection reflection and the desired angle are also called the critical angle of total reflection.

When the light beam hits at an angle greater than the critical angle α, the light beam is completely reflected (ray ´C).

Snell’s law is applied to determine the RI of environment 1. This law states that the ratio of RIs is equal to the inverse ratio of the sine of angles α1 and β1.

If the critical angle α2 = α, then the angle α1 = β1 = 90°. By substituting these values ​​in Snell’s law, the following formula is obtained:

By measuring the RI of medium 2 and the total reflection angle of RI, α of medium 2 is obtained.

This article will continue next week, with the topic of factors affecting the refractive index . Thank you for your attention so far in this tutorial…

In addition, if you have any questions or doubts about the recent article, you can raise them below this article. Our colleagues will answer all your questions with patience and pride.

Factors affecting the refractive index

Dependence of refractive index on wavelength

In almost all cases, the refractive index of different wavelengths is different. Because each material has a specific pattern. This process is called dispersion relation.

Figure 8 shows the absorption rate (blue curve) and the dispersion relation (red curve) of water in a range of different wavelengths. The refractive index in the visible spectrum region is reduced compared to the primary infrared spectrum; While almost no absorption is observed in this range.

Figure 8: Water dispersion curve

To accurately determine the refractive index of a sample, measure the accuracy of the selected wavelength using an interference filter; Although the most refractometric measurements are performed at the wavelength of 589 nm (or 589.3 nm), which corresponds to the wavelength of D-line sodium. In some processes, other wavelengths are also needed.

The historical background of using the standard wavelength of 589nm goes back to the use of sodium lamps. These lamps have been widely available as an affordable and reliable light source. The title “D-line” refers to the spectral line symbol for sodium, which was proposed by a German physicist, Joseph von Fraunhofer, who discovered and mapped more than 570 spectral lines.

Dependence of refractive index on temperature

Temperature is another factor that fundamentally affects the refractive index. Therefore, it should be carefully controlled and measured in all stages. Traditional instruments for temperature control often use a water bath; While modern tools have the option of precise Peltier temperature control. With the help of temperature controller sensors, these devices record the temperature of the sample and the prism with high accuracy.

Figure 9: The refractive index (589.3 nm = ) of water at temperature T compared to vacuum is calculated according to the law of Tilton and Taylor (1938).