Which medium has the highest index of refraction?



The Refractive index (often also as Refractive index is a physical quantity in optics. It characterizes the refraction of an electromagnetic wave at the transition between two media and is the ratio between the phase velocity of light c0 in vacuum and its phase velocity c in the respective medium:

In a substance with a refractive index of 2, the phase velocity of light is exactly half the speed of light in a vacuum, i.e. H. 149,896.229 km / s.

Well-known applications of the determination of the refractive index are the determination of the antifreeze content in coolants (e.g. in liquid-cooled internal combustion engines or in thermal solar systems), the determination of the sugar content of wine or the control of the brake fluid in hydraulic brake systems.

It is also common to have a high refractive index of one "Optically dense medium" or with a low refractive index of one "Optically thin medium" to speak. The term optical density itself should not, however, with the Absorbance be confused.

Physical basics

The term "refractive index" comes from the term refraction and its appearance in Snell's law of refraction. In addition, this physical quantity has no unit and is therefore a dimensionless number. It can also be specified as a complex number:

in which k (ω) is the wavelength-dependent absorption coefficient of the material. With it it is possible to describe the extinction, i.e. the weakening of radiation when it passes through matter.

The refractive index is frequency and thus also wavelength dependent. This effect, known as dispersion, enables, for example, white light to be broken down into its spectral colors on a prism. The frequency dependence of the (complex) refractive index in matter can be described quite well using the model of the Lorentz oscillator.

On the other hand, if you consider the ratio of the group velocities of light in a vacuum to that in the medium, you get the group refractive index

If one wants to theoretically determine the wavelength dependency (dispersion) of the refractive index of a material, one goes over the electrical susceptibility, which records the contributions of the various mechanisms in the material to the properties and ends in the complex permittivity. From here, in the case of (non-magnetic material) directly from the real and imaginary parts of permittivity ε1 and ε2 the sizes n and k to calculate:

Measurement of the refractive index

For the experimental determination of the refractive index of a medium with (e.g. not magnetic) nmed you can z. B. measure the Brewster angle at the transition from air to this medium. In this case applies . A refractometer is used for the measurement.

Other definitions

The definition of the refractive index was done in terms of radiation physics - via the different speeds of light. This equation is elegant, but unsuitable for recently discovered meta-materials because they contain negative refractive indices n occur. This is impossible with this definition.

But the refraction can also be defined in three other ways:

  • via Fermat's principle, according to which light travels the path between two points for which it needs an extreme value of time,
  • on the Huygens-Fresnel principle, which states that every point of a wave front can be viewed as the starting point of a new wave, the so-called elementary wave
  • via the beam optics. According to the aforementioned Snellius law of refraction corresponds to n the sine ratio of the angle of incidence and the angle of reflection. The angle of the incident as well as that of the refracted light beam is related to the perpendicular with respect to the interface.

Refractive index of air and other substances

material Refractive index n (at 589 nm)
vacuum exactly 000000000000001 1
Air (close to the ground) 00000001.000292 1,000292
plasma 0 0 … 1
Cesium[1]000000000000.35 0,35
Airgel00000000001.007 1,007 … 1,24
ice 000000000001.31 1,31
water000000000001.33 1,33
human Eye lens 000000000001.35 1,35 … 1,42
Ethanol[2]0000000001.3614 1,3614
Magnesium fluoride000000000001.38 1,38
Fluorspar (calcium fluoride) 000000000001.43 1,43
human epidermis 000000000001.45 1,45
Carbon tetrachloride000000000001.46 1,46
Quartz glass000000000001.46 1,46
Cellulose acetate (CA) 000000000001.48 1,48
PMMA (Plexiglas ™) 000000000001.49 1,49
benzene000000000001.49 1,49
Crown glass ~000000000001.46 1,46 … 1,65
COC (a plastic) 00000000001.533 1,533
PMMI (a plastic) 00000000001.534 1,534
quartz000000000001.54 1,54
Halite (rock salt) 000000000001.54 1,54
Polycarbonate (PC) 00000000001.585 1,585
Polystyrene (PS) 000000000001.58 1,58
Flint glass ~000000000001.56 1,56 … 1,93
Epoxy resin000000000001.60 1,6
Ruby (aluminum oxide) 000000000001.76 1,76
Glass000000000001.45 1,45 … 2,14
Lead crystal up to 000000000001.93 1.93
Zircon000000000001.92 1,92
sulfur000000000002.00 2
Zinc sulfide[2]000000000002.37 2,37
diamond000000000002.42 2,42
Titanium dioxide (anatase) 000000000002.52 2,52
Silicon carbide000000000002.65 2,65 … 2,69
Titanium dioxide (rutile) 000000000002.71 2,71
Titanium dioxide (rutile, 590 nm) 000000000003.10 3,1
Lead sulfide (PbS, 590 nm) 000000000003.90 3,9

The refractive index of the air averages 1,00029 at sea level. It depends on the density and temperature of the air as well as on the special composition of the air - especially the humidity. Since the air density decreases exponentially upwards - in accordance with the gas laws in a gravitational field, see barometric altitude formula, it is only 1,00011 at an altitude of about 8 km. Nevertheless, the rays of light coming from stars become 0.6 ° near the horizon upscale and in 45 ° by 0.017 °. The effect is called astronomical refraction and affects every terrestrial survey in a similar way.

Since, as described in the introduction, the refractive index of every material depends on the wavelength of the incident light - this also applies to electromagnetic radiation of other wavelength ranges - it would be necessary to specify this as a function of the wavelength (in a table or as a function). Since this is not necessary for many simple applications, the refractive index is usually given for the wavelength of the sodium D-line (589 nm).

Total reflection

If light is not refracted when it strikes a boundary layer between two media with different refractive indices, but is completely reflected, this is referred to as total reflection. In order to produce this effect, the angle (between the perpendicular to the boundary layer and the incident light beam) must exceed a certain value (the acceptance angle or critical angle). For the critical angle of total reflection αG applies

.

application

In chemistry, the refractive index is often used at a certain temperature to characterize liquid substances. The temperature at which the refractive index was determined is added to the symbol for the refractive index, for 20 ° C e.g. B. .

The determination of the refractive index allows a simple determination of the content of a certain substance in a solvent:

Microprocessors are manufactured using photolithography. The etching mask is transmitted by ultraviolet light with a wavelength of 193 nanometers. Usually the smallest possible dimensions are limited by half the wavelength. By using liquids with a refractive index of 1.6, developers at IBM have succeeded in creating a grid of parallel lines with a thickness of just 29.9 nanometers. As a result, a further increase in chip production using the same light source is possible in the future.[3][4]

Refractive index and density

The refractive index of silicate and borosilicate glasses usually increases with their density. For example, lead silicate glasses with a high density also have a high refractive index. It should be noted, however, that despite the general trend, the relationship between refractive index and density is not always linear and that exceptions occur, as shown in the diagram on the right. A relatively high refractive index and low density can be obtained with glasses containing light metal oxides such as Li2Contain O or MgO, while the opposite is achieved with glasses containing PbO and BaO.

Negative refractive indices

In 1964, the Soviet physicist Victor Veselago predicted the existence of materials with negative refractive indices. If the production of such a material were successful, it would be possible to produce lenses with a resolution that would be far better than that of lenses made from conventional optical materials.

Researchers working with Srinivas Sridhar from Northeastern University in Boston succeeded in producing a composite material that contains a fine lattice of metal wires that has a negative refractive index for microwaves. However, until recently it was completely unclear whether and when a material could be produced that also had these properties in the optical field.

In October 2003, a group led by Yong Zhang in Colorado discovered that crystals made of an alloy of yttrium, vanadium and oxygen have a negative refractive index for light waves of a wide frequency range, even without further processing. The crystal consists of two nested crystal lattices with symmetrical optical axes. The negative refraction occurs only in a certain angular range of the angle of incidence. In future experiments, the researchers want to test other suspected properties of negative refraction - such as the reversal of the Doppler effect and Cherenkov radiation.

At the March 2007 meeting of the American Physical Society, Vladimir Shalaev and his colleagues from Purdue University presented a metamaterial with a negative refractive index for radiation in the near infrared range. This means that they are close to the visible spectrum.

In the more distant future, it could be possible to manufacture perfect lenses that can image smaller objects than the diffraction limit of the optics. Researchers led by Prof. Xiang Zhang at the University of Berkeley took a first step in this direction: They used the negative refractive index that occurs in a 35 nanometer thin silver film at the interface with PMMA to build a microscope that has a resolution six times higher than that Wavelength of the light used for observation.[6]

See also

Sources and footnotes

  1. Steffen, H.; Mayer, H .: Optical properties of thin cesium layers in the wavelength range from 0.3 to 0.9 µm and their electrical resistance. In: Zeitschrift für Physik A Hadrons and Nuclei 254 (1972), No. 3, pp. 250-268. doi: 10.1007 / BF01379784
  2. ab Lide, David R .: CRC handbook of chemistry and physics: A ready-reference book of chemical and physical data. 87th ed. Boca Raton Fla. : CRC Taylor & Francis, 2006 - ISBN 0849304873
  3. IBM beats optical lithography limits (Technology News in optics.org of February 22, 2006)
  4. Photolithography is far from over (Newsticker Wissenschaft.de of February 28, 2006)
  5. Glassproperties.com Calculation of the refractive index of glasses (in English)
  6. scientific publication on a super lens (in English)
  • Negative refractive index in microwaves
  • Special crystal with negative refractive index
  • Metamaterial with a negative refractive index in the near infrared range
  • Left Handed Material at Work
  • Refractive indices of different types of glass
  • dynamic worksheets on the subject of "refraction and total reflection"
  • The speed of light is not violated by negative refraction
  • Refractive index measuring stand from Carl Zeiss

Categories: Material Properties | Electrodynamics