Transparency is a physical property that we observe every day, though perhaps it is not one that we give a huge amount of thought to. Also known as diaphaneity or pellucidity, transparency in materials allows light to pass through unaffected, thus making them see-though.
Optically transparent materials are essential in many scientific and manufacturing applications and new ways of utilizing these are being worked on all the time, some of which are highlighted later in the article.
But what makes a material transparent? It is all to do with how the atoms, and hence the electrons, in a material are arranged. If a photon (a particle of light) traveling through a solid meets an electron with an energy gap of equal energy, it will be absorbed by that electron as it 'jumps' to a higher energy level. This means that very little light can travel through the material without being absorbed, thus making the material opaque. However, with transparent materials, the energy gap is larger, so that the photons cannot excite the electrons into a higher energy level. This allows the photons to pass through the material unaffected, making the material see-through. So in essence, the interaction between light and a material is based on the light wavelength and the nature of the material.
This theory is explained in more animated detail by Professor Phil Moriarty for the University of Nottingham.
Why is glass transparent?
Important Transparent Materials
There are many natural and synthetic materials that are transparent, but the ones listed below have some of the most beneficial applications in materials science:
- Glass
- Aluminium oxynitride
- Diamond
- Sapphire
- Transparent ceramics
- Transparent conductive films
Applications of Transparent Materials
The materials listed above have a wide range of applications, from the mundane to the magical.
Flat glass is the most recognizable transparent material, but is used in much more than just windows. Solar panels, microscopes, greenhouses and radiation protection are just some of the further applications of flat glass.
Aluminium oxynitride is used in a number of infrared and defence-related applications such as specialty IR domes, transparent armour, windows for laser communications and also in certain semiconductor-related applications.
The optical properties of diamond ensure that it finds applications in microwave infrared and X-ray research as well as being important in high power laser exit windows.
Sapphire glass find applications in crystal watches, high pressure chambers for spectroscopy and also barcode scanners (since the material's high toughness and hardness make it scratch-resistant).
Transparent ceramics can be used in transparent armor windows, high energy lasers, nose cones for heat seeking missiles, high energy physics, radiation detectors for non-destructive testing, security and medical imaging applications and space exploration.
Materials that are transparent to infrared radiation are often used in high-performance aerospace applications.
Transparent conductive films can be used as electrodes on photovoltaic devices and LEDs. Their conductivity is lower than that of transparent conducting oxides but have low absorption of the visible spectrum allowing them to behave as a transparent conductor.
Photochromic glass finds applications in vehicles, aircrafts, appliances and popular ski-goggles and sun-glasses.
Materials that are transparent to infrared radiation are often used in high-performance aerospace applications.
Innovations in Transparent Materials
Many kinds of glass have been introduced in the construction industry. Thermochromic glass responds to heat and photochromic glass responds to light. By the transmission of battery power or electricity, the transparency of glass partitions or cladding is changed from perfectly clear to completely opaque. This can be done by the transmission of low-voltage electrical charges across a very thin coating on the glass surface that can be activated by sensors that respond to light intensity or manually with a switch. Hence the amount of solar transmission can be controlled enabling reduction in heating or cooling and optimizing artificial lighting.
A current project that makes use of light controlling transparency in an attampt to maximize building energy efficiency is Werner Sobek's experimental house in Germany.
Werner Sobek has attempted to create a building that adapts to different levels of light transmission, ventilation and absorption and has proposed to incorporate mono-functional cells into glass to change their chemistry so that minimum energy is needed to power the building.
Transparent plastic is also extremely important in The Eden project, UK. The largest plant biosphere in the world, The Eden Project makes use of advanced plastic ethyl tetra fluoro ethylene or ETFE pneumatic cushions as cladding. This plastic foil is not just more transparent than glass, these inflatable three layer pillows are much lighter when compared to glass cladding. Furthermore, ETFE does not deteriorate in polluted environmental conditions and is eco-friendly.
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