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Speaker 1: Welcome to Brainstuff, a production of iHeartRadio, Hey, brain Stuff,

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Speaker 1: Lor and Volgevan here. Think about the way of building.

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Speaker 1: Like a house is constructed. You've got outer walls made

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Speaker 1: of materials like brick or wood, with openings built in

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Speaker 1: or cut in for windows, which are frames that hold

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Speaker 1: panes or sheets of glass. Windows make a home feel bright, warm,

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Speaker 1: and welcoming because they let sunlight enter. But why should

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Speaker 1: a glass window be any more transparent than the wood

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Speaker 1: or brick that surrounds it. After all, both materials are solid,

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Speaker 1: and both keep out rain, snow, and wind. Yet wood

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Speaker 1: is opaque and blocks like completely, while glass is transparent

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Speaker 1: and let's sunshine stream through unimpeded. You may have heard

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Speaker 1: some people and even some science textbooks try to explain

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Speaker 1: this by saying that wood is a true solid and

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Speaker 1: that glass is highly viscous liquid. They go on to

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Speaker 1: argue that the atoms in glass are spread farther apart

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Speaker 1: and that these gaps let like squeeze through. They may

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Speaker 1: even point to the windows of centuries old houses, which

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Speaker 1: often look wavy and unevenly thick, as evidence that the

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Speaker 1: windows have flowed over the years, like the slow crawl

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Speaker 1: of molasses on a cold day. In reality, glass isn't

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Speaker 1: a liquid at all. It's a special kind of solid

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Speaker 1: known as an amorphous solid. This is a state of

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Speaker 1: matter in which the atoms and molecules are locked into place,

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Speaker 1: but instead of forming neat orderly crystals, they arrange themselves randomly.

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Speaker 1: As a result, glass is mechanically rigid like a solid,

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Speaker 1: yet it has the disordered arrangement of molecules like liquids.

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Speaker 1: Amorphous solids form when a solid substance like silicon dioxide

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Speaker 1: also known as silica or sand, is melted at high

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Speaker 1: temperatures and then cooled so fast that it doesn't have

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Speaker 1: time to form orderly crystal, a process known as quenching.

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Speaker 1: The panes of glass in old houses aren't wavy and

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Speaker 1: thicker at the bottom because they're still flowing. They were

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Speaker 1: made in a time when glass technology wasn't as good

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Speaker 1: as it is today, so each pain may have had

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Speaker 1: some rippling in it when it's set and been thicker

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Speaker 1: on one end than the other. The carpenter would have

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Speaker 1: put the thicker end on the bottom because it's sturdier

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Speaker 1: like that. In many ways, glasses are like ceramics and

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Speaker 1: have all of the same properties durability, strength and brittleness,

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Speaker 1: high electrical and thermal resistance, and a lack of chemical reactivity. Basically,

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Speaker 1: glass won't corrode, break down, or discolor, which is why

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Speaker 1: it's used in so many applications. What's called soda lime glass,

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Speaker 1: which is the commercial glass that you find in sheet

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Speaker 1: and plate glass glass, jars and bottles and light bulbs,

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Speaker 1: has another important property. It's transparent to the range of

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Speaker 1: electromagnetic wavelengths known as visible light. To understand why, we

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Speaker 1: have to take a closer look at the atomic structure

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Speaker 1: of glass and understand what happens when photons, the smallest

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Speaker 1: particles of light, interact with that structure. Okay, so soda

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Speaker 1: lime glass is made up of mostly silicon dioxide or silica,

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Speaker 1: with a little bit of sodium carbonate or soda ash

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Speaker 1: added for manageability, and calcium carbonate or lime added for

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Speaker 1: hardness and durability. All of those atoms are arranged in

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Speaker 1: the amorphous solid body of the glass. Now, think about

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Speaker 1: the structure of an atom. You've got the atom's nucleus

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Speaker 1: with any protons and neutrons it has, and then the

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Speaker 1: electrons around that occupying different energy levels. If an electron

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Speaker 1: gains energy, it might move to a higher energy level.

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Speaker 1: If it gives up energy, it might move to a

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Speaker 1: lower one. In either case, the electron can only gain

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Speaker 1: or release energy in discrete bundles. Light is made up

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Speaker 1: of photons. Now, let's that are a photon moving toward

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Speaker 1: and interacting with a solid object made up of atoms.

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Speaker 1: One of three things can happen. First scenario, the substance

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Speaker 1: could absorb the photon. This occurs when the photon gives

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Speaker 1: up its energy to an electron located in the material.

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Speaker 1: Armed with this extra energy, the electron is able to

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Speaker 1: skip to a higher energy level while the photon disappears.

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Speaker 1: Second scenario, the substance could reflect the photon. To do this,

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Speaker 1: the photon gives up its energy to the material, but

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Speaker 1: a photon of identical energy is emitted. Third scenario, the

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Speaker 1: substance could allow the photon to pass through unchanged, known

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Speaker 1: as transmission. This happens because the photon doesn't interact with

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Speaker 1: any electrons and continues its journey until it interacts with

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Speaker 1: another object. Clear glass falls into this last category. Photons

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Speaker 1: pass through the material because they don't have sufficient energy

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Speaker 1: to excite a lo electrons in the glass to a

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Speaker 1: higher energy level. Physicists sometimes talk about this in terms

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Speaker 1: of band theory, which says energy levels exist together in

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Speaker 1: regions known as energy bands. In between those bands are

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Speaker 1: regions known as band gaps, where energy levels for electrons

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Speaker 1: don't exist at all. Some materials have larger band gaps

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Speaker 1: than others. Glass has pretty large band gaps, which means

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Speaker 1: it's electrons require much more energy before they can skip

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Speaker 1: from one energy band to another and back again. Photons

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Speaker 1: of visible light, that is, light with wavelengths from four

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Speaker 1: hundred to seven hundred nanimeters, corresponding to the colors violet, indigo, blue, green, yellow, orange,

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Speaker 1: and red, these photons simply don't have enough energy to

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Speaker 1: cause this skipping in glass. Therefore, photons of visible light

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Speaker 1: travel right through glass instead of being absorbed or reflected,

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Speaker 1: thus making glass transparent. At wavelengths smaller than visible light,

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Speaker 1: photons begin to have enough energy to move the electrons

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Speaker 1: in glass from one energy band to another. For example,

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Speaker 1: ultraviolet or UV light, which has a wavelength ranging from

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Speaker 1: ten to four hundred animeters, cannot pass through most sodo

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Speaker 1: lime glass, such as the glass and a window pane.

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Speaker 1: This makes a window, including the window in our hypothetical house,

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Speaker 1: as opaque to UV light as wood is to visible light,

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Speaker 1: which is pretty excellent considering the damage that EV light

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Speaker 1: can do to where eyes and skin. Pure silica glass

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Speaker 1: would let UV light through. There are other types of

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Speaker 1: transparent glass too, like lead glass, which is sometimes called

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Speaker 1: lead crystal and is used in decorative pieces because it

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Speaker 1: has such high brilliance, especially when cut with lots of facets.

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Speaker 1: There's also borsilicate glass, which is used for kitchen and

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Speaker 1: lab equipment because it's much better at withstanding temperature changes

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Speaker 1: than lead or soda lime glasses. It actually took humans

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Speaker 1: thousands of years to work out how to make glass clear.

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Speaker 1: Ancient crafts people in Mesopotamia in Egypt some four thousand

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Speaker 1: years ago discovered sodoaline glass and cast it to make

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Speaker 1: objects like beads and hollow containers. However, natural impurities and

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Speaker 1: the raw materials for glass will cause it to turn colors.

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Speaker 1: For example, iron oxides will create a blue to green

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Speaker 1: to yellow to brownish tint. By around the first century

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Speaker 1: bceeb artisans in Jerusalem invented the technique of blowing glass

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Speaker 1: very thinly so that it appeared transparent mostly, but it

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Speaker 1: wasn't until the fourteen hundreds that Venetian glass makers figured

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Speaker 1: out how to make glass colorless. They achieved this by

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Speaker 1: carefully controlling the purity of their raw materials and by

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Speaker 1: adding small amounts of other materials to counteract the tints

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Speaker 1: caused by iron oxides like antimony, potash, or manganese oxides.

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Speaker 1: This glass was considered super fancy and also led to

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Speaker 1: the development of technologies like magnifying lenses. All along this

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Speaker 1: timeline and up through today, people working with glass have

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Speaker 1: developed different techniques to color glass on purpose. Modernly, that's

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Speaker 1: often by adding metals or metal oxides alike copper for

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Speaker 1: red glass or cobalt for blue. When they do, the

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Speaker 1: glass will start reflecting those particular wavelengths of light. Today's

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Speaker 1: episode is based on the article what makes glass Transparent

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Speaker 1: on how Stuffworks dot Com, written by William Harris. Rain

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Speaker 1: Stuff is production of by Heart Radio in partnership with

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Speaker 1: how Stuffworks dot Com, and it's produced by Tyler Klang.

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Speaker 1: For more podcasts from My Heart Radio, visit the iHeartRadio app,

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Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.

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Speaker 1: M