Optical properties are the beating heart of glass technology [1–5]. Imagine the amazement of early technologists who discovered transparent, solid droplets in the ashes of their fires—sand grains whose light scattering boundaries had surrendered their identity to a uniform melt. Over time they trialled new raw materials, optimising proportions, processing and properties. They found minerals that added colour and that the magical combination of colour and transparency mimicked precious stones. New industries supported an international trade in coloured beads. The Romans extended these skills, generating artwork worthy of its place in today’s international museums [6]. From 1000 AD, throughout Europe cathedrals appeared with stained glass windows protecting from weather, admitting light and illuminating bible stories with images readable by rich and poor alike. Later, coloured bottles signified ownership, warned of dangerous contents or protected from damaging UV radiation, while traffic lights and airport runway lighting using coloured glass filters created safer travel [7].

Off-colours naturally suggested poor quality. The Romans sourced the purest sands for fine tableware and knew that Mn-rich minerals reduced its green hue; they didn’t know the cause: a redox Fe²⁺/Mn³⁺ interaction on cooling (reversible in strong sunlight!).

Of course, glass transparent in the visible nevertheless absorb IR and UV light. The greenhouse effect arises from the easy transmission of sunlight through glass which then traps the longer wavelengths emitted by the cooler objects inside. Controlling this helps minimise our need for heating in winter and cooling in summer. Now, smart glazing with selected coatings (low-e, solar control films) helps achieve it. [1,8]

In the last 2 centuries rare earth elements (REs) have added to the palette of available colours; the key energy levels are deeply embedded f orbitals, not the d orbitals of transition metals. So, the host’s structure affects the RE ion’s optical characteristics less. Slower energy transfer from excited ions to lattice vibrations means states with longer lifetimes, even more so if the host glass has heavy, weakly bonded ions with slow natural vibrations, poorly matched to the optical transitions. Precipitating nanocrystals that sequester RE ions, give added control. Many devices now use such materials: lasers, white light sources, upconverters (absorbing IR to emit visible).

Amazingly the Romans made some coloured glasses with colloidal precipitates. Non-absorbing particles half a wavelength across scattered light everywhere, creating white ‘opalescent’ ware. Smaller or larger particles scattered less. Red, orange and yellow glasses often use nanoparticles of Au, Ag, Cu, Se or Cd(S,Se). Light is absorbed at specific wavelengths by exciting surface plasmon oscillations (metals) or band gap transitions (semiconductors). Larger particles both absorb and scatter. The Lycurgus Cup (British Museum) is a fine example of a dichroic glass displaying different colours in transmitted (back lit) and scattered light (front lit) [9].

In certain Ag containing glass ceramics, a single photon with sufficient energy can initiate electron transfer between Ce³⁺ and Ag⁺ ions to give Ag° and Ce⁴⁺. These Ag atoms easily agglomerate, forming nucleating agents. Microcrystalline images can be developed by heating. One product gave a soluble crystalline phase in exposed areas, allowing optical machining of complex shapes such as cochlea implants.

Stained glass, created by diffusing Ag⁺ ions into a glass surface where they are reduced (e.g. by Fe²⁺) to form Ag nanospheres, is yellow - the Ag plasmon resonant frequency is in the blue. Corning made tableware containing dendritic silver halide particles whose tips could be photographically developed to give silver needles of varying aspect ratios according to exposure time. This shape change altered the natural plasmon oscillation frequencies, creating absorptions over different wavelengths and giving a range of vibrant colours.

From the early 20th century radios waves were used to carry information. This required electronic valves, sealed glass envelopes with a stable vacuum inside. A perceived extension of radio was to encode light, its higher frequency offering a much higher information density. While sending light pulses through the atmosphere was acceptable for lighthouses communicating with ships it was impractical for telephone calls. A secure medium was needed. Glass was considered but its transparency was questioned. The adoption of a modified vapour phase method used by the developing electronics industry for deposition of pure/doped layers of silicon, meant (silica) glass with purities of ppb or less was possible, giving transmission over many kilometres.

So far, our focus has been absorption. Light refraction on passing from air to glass is also important. Smooth (parallel) surfaces are needed for distortion-free transmitted/reflected images; internal defects (bubbles, crystals) must be avoided. Jewellery and containers preceded glass windows for this reason. Glassmaking involved either pulling a rod from the melt surface or blowing bubbles. The simplest way to make a window was to blow a bubble in a square mould and cut out the walls; such windows were small and the lensing effect comparable to old, thick spectacles. Later the tricks of the trade allowed a bubble to be extended to a cylinder, which could be opened into a sheet, or spun into a flat disc, but both still caused optical distortion - the surfaces were never flat nor parallel. In 1665, Colbert invented cast flat glass in France which led to the first multinational glass company (Saint-Gobain); the glass sheets had better quality, but still needed mechanical polishing to remove distortion. Not until the 20^th century were mechanised drawing processes developed; polishing using large rotating wheels persisted for decades. The Float process had to await the middle of the 20^th century. This breakthrough technology let molten glass spread across molten tin - the bottom surface was formed on the liquid tin, and the top surface was untouched, creating 2 flat, parallel surfaces that earlier glass makers could only dream of. Development never stops; ultrathin flat glass for mobile phone covers is now made by letting molten glass overflow both sides of a platinum boat; the two streams join underneath giving 2 untouched outer surfaces.

Investigators studying light bending at interfaces coined the term refractive index. While first used by Young (1807), Newton and others understood such concepts. Newton, in a darkened room with a prism and a sunbeam realised that this ‘refractive index’ depended on colour. In the middle of the 19^th century scientists such as Abbe, Schott and Zeiss studied the dependence of refractive index and its dispersion on composition. The coincidence of a chemist, a glassmaker and a manufacturer of optical systems in Jena contributed dramatically to a great leap forward in optics using new glasses of improved quality. An advantage of the glassy state is its ability to accommodate different components and so adjust properties. This was vital in creating compound lenses able to eliminate aberrations associated with dispersion.

Earlier observers realised that non-flat surfaces refracted light in useful ways. An early device was the hemispherical magnifying ruler used by older monks to read their beautifully created illuminated bibles. The step from a magnifying hemispherical cylinder to a spherical lens made by mechanical grinding soon followed; the first painted image of a monk wearing spectacles dates back 670 years. The ability to focus light sparked a new era of optics: telescopes, microscopes, binoculars and later, cameras. A related application was the use of a bowl filled with water (a Tailor’s Bowl) to focus candlelight so sewing could continue beyond daylight hours. Now geology, biology, forensic science and astronomy rely on sophisticated optical devices. Early Astronomers realised that the planets circle the sun; their observations demanded analysis by mathematicians and were the basis of Newtonian gravitational theory. Observations of bacteria, diseased joints and body organs has pushed back the boundaries of medicine. And early photographs were recorded on glass plates.

Temperature also affects profile so zero expansion glass ceramics are used for huge reflecting telescopes. The Hubble and the newest James Webb astronomical telescopes are exploring the first moments after the Big Bang, 13 billion years ago, discovering the most distant stars and exploring the mysteries of black holes.

19^th century physicists realised that light is an electromagnetic wave exhibiting polarisation, interference and diffraction, although the first recorded observations of polarisation involved calcite crystals (1669). Understanding how stresses in glass affected polarisation helped scientists measure and understand annealing. They also realised that reflected light is polarised. Monomode optical fibres too, have not one but two propagating waves, polarised at right angles, explaining bending losses.

Antireflection coatings are a quarter wavelength thick; destructive interference between reflections from the top surface and the interface reduces reflection losses. Two or more coatings also allows compensation of refractive index dispersion.

Diffraction is the aim of gratings burned into optical fibres using 2 intense intersecting excimer laser beams to induce optical defects and sinusoidal changes in refractive index. As Bragg reflectors at a specific wavelength, they allow multiplexed optical signals to be separated into component wavelengths. Determining the optimum refractive index profile across an optical fibre relies on Maxwell’s electromagnetic radiation equations and is tailored to give monomode, zero dispersion operation with zero light leakage. Indeed, refractive index-dispersion can also be compensated. Glass’s success as a transmitter of information means that we are all connected to an extent unimaginable 50 years ago; indeed, not only to each other but also to information stores through the glass screens of our mobile phones. Internet evolution to 5G is supported by transforming optical fibres to photonic structured fibres permitting the information transported per second to grow exponentially, improving the world economy and our lifestyle as demonstrated during the COVID pandemic.

The 20^th century saw quantum physics drive our understanding of electronic energy levels, optical absorption, refractive index, dispersion and optical non-linearities. New devices appeared: lasers; monomode optical fibres; hollow core fibres; optically connected computer chips; information storage devices. New fluoride glasses have allowed optical fibres access to mid IR applications; chalcogenide glasses (S, Se, Te, etc.) are transparent at even longer wavelengths. The latter have small band gaps and their photostructural properties are another bonus. Understanding their electronic structure has generated at least one Nobel Prize. Their properties have led to xerography, switchable memory devices, optical communications and more [10].

Glass impacts on food; energy generation and conservation; the sciences, medicine and health, even social welfare. Its story dates back 3500 years, linking problems, opportunities and solutions, and stretches to distant horizons. This same rich history and amazing potential kick-started a discussion in the US, after the UN supported an International Year of Light. Senior figures from Industry and Academia: David L. Morse, Jeffrey W. Evenson and Charles L. Craig, L David Pye and Manoj Choudhary debated whether we were entering an Age of Glass and indeed whether to promulgate this concept. From there the ICG led by Prof Alicia Duran agreed to test the water and apply formally to the United Nations that 2022 be declared an International Year of Glass. The main thread of the argument was 1) that glass had much to celebrate and 2) that glass and its products aligned perfectly with many of the UN’s 2030 Humanitarian Goals; the proposal was accepted in May 2021. Appropriately 2022 is the anniversary of many glass events. Importantly the glass community internationally has engaged in: creating numerous celebrations e.g. the recent US National Glass Day; in encouraging communication, in writing books [1]; and in making videos that will remain useful long after. You can find more on the IYOG web site [11] including links to local web sites. Please join in.