An optical glass window is a piece of glass that has been specially manufactured for use in optical applications. Optical glass is made from a specific type of silica sand, which is heated and cooled very slowly to create a very pure, homogeneous glass.
This type of glass has very low levels of impurities, which makes it ideal for use in optical applications where light needs to be transmitted with minimal distortion. Optical glass is also very resistant to thermal shock, meaning it can withstand sudden changes in temperature without shattering. Glass windows are used in a wide range of optical devices, including telescopes, cameras, and binoculars. They are also used in some types of medical equipment, such as x-ray machines.
Glass is not just a transparent solid material. Glass is a very homogeneous material among many solid materials and has isotropic properties due to its random structure. In addition, being an inorganic material, it has high durability, which makes it different from other transparent materials. The characteristics can be continuously tuned by designing its composition, in other words, by changing the ratio of various elements. Optical glass is a glass material developed by taking full advantage of these characteristics and is used in various optical components including lenses.
Glass has a long history and was already being used by mankind about 5,000 years ago. At first, it was valued as jewelry because it has clear transparency, develops a vivid color, and shines when exposed to light. The brightness and color of glass are due to the characteristics of glass, such as its transparency and refractive index, and optical glass makes use of these characteristics. Transparency and refractive index are two important parameters of optical glass. They will be explained later.
At present, an understanding of the physical properties of glass has been deepened, and in addition to the indices of optical properties, solubility at high temperatures and formability when softened are also taken into account when designing glass. For example, various glass products around us are mainly made from silica sand, which is mainly composed of silica (SiO2). Silica sand melts at a very high temperature (over 1,700°C). So, soda ash (Na2CO3) is usually added to lower the melting point and lime (CaO) is added to the ingredients to make the glass insoluble in water. This glass is called soda-lime glass and is often used in windows. In this way, we can make glass having various characteristics by adding other elements to the predominant ingredient, silica.
Glass network structure at the atomic level
Silica has a strong covalent bond of Si-O, but when additives such as Na are added, the covalent bond breaks and the properties change. Atoms in the network structure are arranged randomly with no regularity like a crystal, and such a structure is called amorphous. For this reason, whatever direction light goes in glass, it propagates through homogeneous crystal fields, and such high homogeneity is one of the important characteristics of optical glass.
When you look at some scenery through glass, you can see things on the other side clearly. This is because the glass transmits the visible light that has passed through an object. Visible light is a form of electromagnetic wave with a wavelength normally in the range of 380 nm to 780 nm that can be recognized by the human eye. Various wavelengths are used in the optical field. For example, ultraviolet light is used for sterilization, near-infrared light is used for sensing and optical communication, and far-infrared light is used for thermal cameras and night vision cameras. When we say optical glass is transparent, that means the glass has high transmittance in the wavelength range of the relevant application.
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Wavelength ranges of electromagnetic waves and their general names.
Light is a form of electromagnetic wave, and each wave has a wavelength (the length of a period of an electromagnetic wave). Human eyes can generally sense only light in the wavelength range called visible light rays, but optical glass and its processed products handle a wide range of light rays (electromagnetic radiation), including X-rays and microwaves, depending on the application.
The transparency of glass depends on the composition of the element used in the glass. In addition to that, it is important to control absorption and the scattering of light to achieve high transparency. For example, ordinary window glass contains iron in its composition, so it is slightly green even though it is transparent. This is due to the absorption of iron ions in the glass. Optical glass is designed to reduce such absorption by impurities. Even micron-size foreign matters inside the glass scatter light rays there, reducing the transparency of the glass. Some types of optical glass actively utilize absorption and scattering caused by additives and impurities.
Conceptual image of light rays passing through glass.
Light entering glass is first partially reflected on the surface and the rest enters the interior. While it is propagating through the glass, if additives that absorb light are present in the glass, part of the light is absorbed, and if additives or imprities that scatter light are present, the light is scattered to random directions. When the light reaches the opposite side, part of the light is reflected again, and the remaining light comes out of the glass as transmitted light.
Refractive index is one of the most basic indices of optical properties of glass, along with transparency. The refractive index of glass is determined by its composition. As its name implies, it influences the refractive angle of the light. In addition to that, reflectance and transmittance of the glass are also dependent on the refractive index. The refractive index is the most fundamental characteristic that indicates the interaction between light and the glass and is the basis of the optical behavior of glass.
The most familiar application of glass substrates with different refractive indices is the camera lens unit. Recently, optical waveguide substrates for AR/MR glasses have also been commercialized using the high optical confinement properties of high-refractive-index glass.
&#; Refraction condition (air to glass)
&#; Refraction condition (glass to air)
&#; Total reflection condition
Refraction and total reflection of light traveling between different media (e.g., air and glass).
When a light ray travels from the air to the inside of glass or from the inside of glass to the air, it does not go straight but changes its direction of travel, and this phenomenon is called refraction. The refractive index of a material is expressed as the ratio of the change in angle that happens when a light ray enters the material from a vacuum; the higher the refractive index, the greater the angular change. When a light ray travels from air with a low refractive index to glass with a high refractive index, the angle of the ray going out of the interface (refraction angle) is smaller than the angle of the ray coming into the interface (incidence angle). On the other hand, when a light ray travels from glass with a high refractive index into air with a low refractive index, the refraction angle becomes larger than the incidence angle. If the incidence angle of a light ray gets larger when it travels to air with a low refractive index from glass with a high refractive index, the refraction angle reaches 90°; that is, the light ray is totally reflected without entering the air. The angle at which this total reflection occurs is called the critical angle. The higher the refractive index of the glass, the smaller the critical angle and the wider the angle range of total reflection. That means, the glass has a strong light-confinement effect.