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I’d love it if someone can direct to an explanation accessible to a non physics college graduate that actually explains why glass (and other things like quartz

I’d love it if someone can direct to an explanation accessible to a non physics college graduate that actually explains why glass (and other things like quartz and even water/air) are transparent, how exactly does light pass through them that also makes them that causes refraction and what exactly makes them reflect light as well.

Gigachad 4y ago
I have seen a few casual explanations of it and I still don’t really understand it.
s0rce 4y ago
Things are transparent that don't absorb, scatter or reflect the light. For stuff like quartz the band gap (energy above which photons get absorbed) is higher than the energy of visible light so the light just goes through and its transparent... Are you asking why certain materials don't absorb visible light?
analog31 4y ago
The oscillating electric field of the light wave induces a sympathetic oscillation of the electric charges bound to atoms or molecular bonds in the material. The degree to which those charges can align themselves to the field is a characteristic of each material. Taking that polarizability into account in Maxwell's Equations yields a wave equation for electromagnetism, just like it does in a vacuum, but with a different propagation velocity, slower than the speed of light. In a vacuum, there's a corresponding polarizability of free space, related to the speed of light. In undergrad electrodynamics, prior to learning about relativity or quantum, we didn't try to explain it any further than that. It's transparent because the electrons stay attached to their atoms. If they could break free, then conduction would occur, and another solution of Maxwell's equations shows the wave being quenched. You could imagine conduction causing loss of energy via heating, like the way an electric stove works. If the electrons could absorb energy by making a quantum transition, some of energy could be released as heat rather than light. Refraction and reflections could be thought of as necessary consequences of continuity at an interface between materials of different composition. A number of things related to electric and magnetic field vectors have to remain the same at both sides of the boundary. This is akin to a conservation law, and requires introduction of a third wave at the boundary -- the reflected wave -- to make everything balance. Another way to look at refraction is via Huygen's Principle, where each point on a wave front is a source of a spherical wave front, and accounting for the different speeds in the two materials (e.g., air and glass) gives the law of refraction. And there's even yet another explanation which can be found by figuring out the path that the light takes, which results in a minimum for the time of travel. It's interesting that the law of refraction was worked out before electromagnetism was understood. I'm an industrial physicist, and this is hand-wavy, but I'm trying for an explanation that doesn't require equations.
adrian_b 4y ago
In general, for the substances that do not contain atoms/ions/molecules which absorb the visible light (i.e. most substances which contain neither transitional metals nor organic molecules with multiple bonds, and which are neither metals nor semi-metals, which have free electrons, nor semiconductors with very small bandgap, where the visible light creates free electrons), the condition to be transparent is that the substance must be in one of the following states: gas, liquid, glass (i.e. solid obtained by very fast cooling) or single-crystal (i.e. solid obtained by very slow cooling). The reason is that in all these states, where the substance is fluid, amorphous or a single crystal, the substance is homogeneous, all the points inside it have the same properties, there are no internal boundaries, so light propagates in straight lines inside the substance, so light can pass through the substance, with only a pair of refractions on the external surface, when the light enters and exits the substance. Thus the substance is transparent. Most solids, when they are neither cooled so fast to form an amorphous glass nor cooled so slow to form a single crystal, form at cooling a polycrystalline solid, e.g. like any rock or stone, which is made of a huge number of very small crystal grains. In a polycrystalline solid, there is a huge number of internal boundaries, which refract and reflect the light. Therefore the light cannot propagate in straight lines, but in extremely long and intricate trajectories. If the substance is extremely pure, so nothing absorbs the light, it will be translucent, i.e. the light eventually gets out but you can not see any image of what is behind. Normal substances have some impurities that absorb the light. Due to the extremely long path of the light inside a polycrystalline substance, it will be eventually absorbed, so the substance is opaque. So the difference between air, water, common glass, and quartz crystals versus most solids (which are neither amorphous nor single crystals) is that the former do not have internal boundaries, so the propagation of light is affected only at the external surface, while the latter have a huge number of internal boundaries, which reflect and refract the light. The reason why the light is refracted when passing through a boundary between two different media can be understood without any knowledge about the nature of light, except that it behaves like a wave. The light waves have different speeds on the 2 sides of the boundary. From the condition of continuity in time at a point on the boundary it results that the frequency of the waves must be the same in both sides, therefore the wavelengths must be different. From the condition of the continuity of the wavelength measured along the boundary on its 2 sides, it results that there must be an angle between the wave propagation directions on the 2 sides. To explain why there is also reflection at a boundary, that is more complex, because the explanation needs some model of how light is absorbed and emitted and that depends on the chemical nature of a substance (some, like metals, reflect better than others). Some simplified explanation is at https://en.wikipedia.org/wiki/Reflection_(physics)#Mechanism https://en.wikipedia.org/wiki/Reflection_(physics)#Mechanism

Gigachad 4y ago

I have seen a few casual explanations of it and I still don’t really understand it.

s0rce 4y ago

Things are transparent that don't absorb, scatter or reflect the light. For stuff like quartz the band gap (energy above which photons get absorbed) is higher than the energy of visible light so the light just goes through and its transparent... Are you asking why certain materials don't absorb visible light?

analog31 4y ago

The oscillating electric field of the light wave induces a sympathetic oscillation of the electric charges bound to atoms or molecular bonds in the material. The degree to which those charges can align themselves to the field is a characteristic of each material. Taking that polarizability into account in Maxwell's Equations yields a wave equation for electromagnetism, just like it does in a vacuum, but with a different propagation velocity, slower than the speed of light. In a vacuum, there's a corresponding polarizability of free space, related to the speed of light. In undergrad electrodynamics, prior to learning about relativity or quantum, we didn't try to explain it any further than that. It's transparent because the electrons stay attached to their atoms. If they could break free, then conduction would occur, and another solution of Maxwell's equations shows the wave being quenched. You could imagine conduction causing loss of energy via heating, like the way an electric stove works. If the electrons could absorb energy by making a quantum transition, some of energy could be released as heat rather than light. Refraction and reflections could be thought of as necessary consequences of continuity at an interface between materials of different composition. A number of things related to electric and magnetic field vectors have to remain the same at both sides of the boundary. This is akin to a conservation law, and requires introduction of a third wave at the boundary -- the reflected wave -- to make everything balance. Another way to look at refraction is via Huygen's Principle, where each point on a wave front is a source of a spherical wave front, and accounting for the different speeds in the two materials (e.g., air and glass) gives the law of refraction. And there's even yet another explanation which can be found by figuring out the path that the light takes, which results in a minimum for the time of travel. It's interesting that the law of refraction was worked out before electromagnetism was understood. I'm an industrial physicist, and this is hand-wavy, but I'm trying for an explanation that doesn't require equations.

adrian_b 4y ago

In general, for the substances that do not contain atoms/ions/molecules which absorb the visible light (i.e. most substances which contain neither transitional metals nor organic molecules with multiple bonds, and which are neither metals nor semi-metals, which have free electrons, nor semiconductors with very small bandgap, where the visible light creates free electrons), the condition to be transparent is that the substance must be in one of the following states: gas, liquid, glass (i.e. solid obtained by very fast cooling) or single-crystal (i.e. solid obtained by very slow cooling). The reason is that in all these states, where the substance is fluid, amorphous or a single crystal, the substance is homogeneous, all the points inside it have the same properties, there are no internal boundaries, so light propagates in straight lines inside the substance, so light can pass through the substance, with only a pair of refractions on the external surface, when the light enters and exits the substance. Thus the substance is transparent. Most solids, when they are neither cooled so fast to form an amorphous glass nor cooled so slow to form a single crystal, form at cooling a polycrystalline solid, e.g. like any rock or stone, which is made of a huge number of very small crystal grains. In a polycrystalline solid, there is a huge number of internal boundaries, which refract and reflect the light. Therefore the light cannot propagate in straight lines, but in extremely long and intricate trajectories. If the substance is extremely pure, so nothing absorbs the light, it will be translucent, i.e. the light eventually gets out but you can not see any image of what is behind. Normal substances have some impurities that absorb the light. Due to the extremely long path of the light inside a polycrystalline substance, it will be eventually absorbed, so the substance is opaque. So the difference between air, water, common glass, and quartz crystals versus most solids (which are neither amorphous nor single crystals) is that the former do not have internal boundaries, so the propagation of light is affected only at the external surface, while the latter have a huge number of internal boundaries, which reflect and refract the light. The reason why the light is refracted when passing through a boundary between two different media can be understood without any knowledge about the nature of light, except that it behaves like a wave. The light waves have different speeds on the 2 sides of the boundary. From the condition of continuity in time at a point on the boundary it results that the frequency of the waves must be the same in both sides, therefore the wavelengths must be different. From the condition of the continuity of the wavelength measured along the boundary on its 2 sides, it results that there must be an angle between the wave propagation directions on the 2 sides. To explain why there is also reflection at a boundary, that is more complex, because the explanation needs some model of how light is absorbed and emitted and that depends on the chemical nature of a substance (some, like metals, reflect better than others). Some simplified explanation is at https://en.wikipedia.org/wiki/Reflection_(physics)#Mechanism https://en.wikipedia.org/wiki/Reflection_(physics)#Mechanism