What is glass?
Glass is an amorphous
(non-crystalline) solid material. Glass is commonly used for windows, bottles,
modern hard drives or eyewear and examples of glassy materials include soda-lime
glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium
oxynitride. The term glass (late-Latin ‘glaesum’) developed in
the late Roman Empire. In Germany that term was used for glossy and transparent
amber, which was mainly used - as early glass was too - in jewels.
Strictly speaking, a glass is defined as an inorganic product of fusion which
has been cooled through its glass transition to the solid state without crystallizing.
Many glasses contain silica (quartz sand) as their main component and glass
former.
The optical and physical properties of glass make it suitable for applications such as flat glass, container glass, optoelectronics material, laboratory equipment, thermal insulator (glass wool), reinforcement fiber (glass-reinforced plastic, glass fiber reinforced concrete), art and - off course - optics. But, unfortunately, not all glass can be used for photography. The mixture of various components - often a well-kept secret - is very important. From its start in 1917 Nikon is able to melt its own glass; nowadays from a wide choice of nearly 300 different components and/or ingredients.
Glass ingredients.
Pure silica (SiO2) has a "glass melting point" of over 2300 °C (4200 °F). While pure silica can be made into glass for special applications other substances are added to common glass to simplify processing. One is sodium carbonate (Na2CO3), which lowers the melting point to about 1500 °C (2700 °F) in soda-lime glass; "soda" refers to the original source of sodium carbonate in the soda ash obtained from certain plants. However, the soda makes the glass water soluble, which is usually undesirable, so lime {calcium oxide (CaO), generally obtained from limestone}, some magnesium oxide (MgO) and aluminium oxide (Al2O3) are added to provide for a better chemical durability. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass. It is, however, not suitable for photographic lenses.
As well as soda and lime, most common glass has other ingredients added to change its properties. Lead glass, such as lead crystal or flint glass, is more 'brilliant' because the increased refractive index** causes noticeably more "sparkles", while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion, and was formerly used in producing high-quality lenses (e.g. Canon 1.2/55 mm.), but due to its radioactivity it has been replaced by lanthanum oxide in modern glasses. Nikon’s famous ED-glass (ED = extra dispersion) has a high content of lanthanum oxide. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium (IV) oxide can be used for glass that absorbs UV wavelengths (biologically damaging ionizing radiation).
In August 2010 Nikon introduced - with the wide-range Zoom-Nikkor 4.5-5.6/55-300 mm. - a new type of optical element: HRI (High Refractive Index) element which "offers the same benefits as multiple elements made of standard optical glass" as Nikon's press release explains. "It compensate plane curvature and spheric aberration", without mentioning whether this element is made of glass or some other material.
Finally, fining agents such as sodium sulfate, sodium chloride, or antimony
oxide are added to reduce the bubble content in the glass.
Flint & Crown
In most older and modern lenses flint glass and crown glass are widely used.
Flint glass is
optical glass that has relatively high refractive index and low Abbe number*.
Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55
or less. The currently known flint glasses have refractive indices ranging
between 1.45 and 2.00.
With respect to glass, the term flint derives from the flint nodules found
in the chalk deposits of southeast England that were used as a source of high
purity silica by George Ravenscroft, circa 1662, to produce a potash lead
glass that was the predecessor to English lead crystal.
Traditionally, flint glasses were lead glasses containing around 4–60%
lead oxide; however, the manufacture and disposal of these glasses are sources
of pollution. In many modern flint glasses, the lead can be replaced with
other additives such as titanium dioxide and zirconium dioxide without significantly
altering the optical properties of the glass.
Flint glass can be fashioned into rhinestones which are used as diamond simulants.
Crown glass is
optical glass and is produced from alkali-lime (RCH) silicates containing
approximately 10% potassium oxide. It has low refractive index (1.52) and
low dispersion (with Abbe numbers around 60).
As well as the specific material named crown glass, there are other optical
glasses with similar properties that are also called crown glasses. Generally,
this is any glass with Abbe numbers in the range 50 to 85. For example, the
borosilicate glass Schott BK7 is an extremely common crown glass, used in
precision lenses. Borosilicates contain about 10% boric oxide, have good optical
and mechanical characteristics, and are resistant to chemical and environmental
damage. Other additives used in crown glasses include zinc oxide, phosphorus
pentoxide, barium oxide, and fluorite.
A concave lens
of flint glass is commonly combined with a convex lens of crown glass to produce
an achromatic doublet. The dispersions of the glasses partially compensate
for each other, producing reduced chromatic aberration compared to a singlet
lens with the same focal length.
Scientists, like the English theoretical physicist Sir John Brian Pendry (the
science of invisibility), are able to create ‘super lenses’ or
‘hyper lenses’ containing ‘metamaterial’.
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*In physics and
optics, the Abbe number, also known as the V-number or constringence (aka
reciprocal dispersion) of a transparent material, is a measure of the material's
dispersion (variation of refractive index with wavelength) in relation to
the refractive index. It is named for Ernst Abbe (1840–1905), the German
physicist who defined it.
**A refractive index (or index of refraction) (first noticed by Willebrord
Snell van Royen aka Snellius - Netherlands-1580-1626) is a measure of how
much the speed of light (or other waves such as sound waves) is reduced inside
the medium. For example, typical soda-lime glass has a refractive index close
to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed
of light in a vacuum. Two common properties of glass and other transparent
materials are directly related to their refractive index. First, light rays
change direction when they cross the interface from air to the material, an
effect that is used in lenses. Second, light reflects partially from surfaces
that have a refractive index different from that of their surroundings.
The refractive index of miscellaneous materials:
water = 1.33 / gelatin = 1.54 / fused silica = 1.46 / soft crown glass = 1.52 / barium light flint glass = 1.60 / lanthanum glass = 1.68-1.88 / silver chloride = 2.06 / diamond = 2.42
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