To move from a lower to a higher energy level, an electron must gain energy. Oppositely, to move from a higher to a lower energy level, an electron must give up energy. In either case, the electron can only gain or release energy in discrete bundles. Now let's consider a photon moving toward and interacting with a solid substance. One of three things can happen:. Glass, of course, falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a glass electron to a higher energy level.
Physicists sometimes talk about this in terms of band theory , which says energy levels exist together in regions known as energy bands. In between these bands are regions, known as band gaps , where energy levels for electrons don't exist at all.
Some materials have larger band gaps than others. Glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another and back again. Photons of visible light — light with wavelengths of to nanometers, corresponding to the colors violet, indigo, blue, green, yellow, orange and red — simply don't have enough energy to cause this skipping.
Consequently, photons of visible light travel through glass instead of being absorbed or reflected, making glass transparent. At wavelengths smaller than visible light, photons begin to have enough energy to move glass electrons from one energy band to another. For example, ultraviolet light, which has a wavelength ranging from 10 to nanometers, can't pass through most oxide glasses, such as the glass in a window pane.
This makes a window, including the window in our hypothetical house under construction, as opaque to ultraviolet light as wood is to visible light.
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Mobile Newsletter chat avatar. Connect and share knowledge within a single location that is structured and easy to search. Once I asked this question from my teacher and he replied "Because it passes light. The same question again, Why glass is transparent? Why does light pass through it, while for opaque objects, it does not?
Photons pass through glass because they are not absorbed. And they are not absorbed because there is nothing which "absorbs" light in visual frequencies in glass. You may have heard that ultraviolet photons are absorbed by glass, so glass is not transparent for them.
Exactly the same happens with X-rays for which our body is nearly transparent whilst a metal plate absorbs it. This is experimental evidence.
Any photon has certain frequency - which for visible light is related to the colour of light, whilst for lower or upper frequencies in the electromagnetic spectrum it is simply a measure of the energy transported by photon. A material's absorption spectrum which frequencies are absorbed and how much so depends on the structure of the material at atomic scale. Absorption may be from atoms which absorb photons remember - electrons go to upper energetic states by absorbing photons , from molecules, or from lattices.
There are important differences in these absorption possibilities:. As glass is a non-crystalline, overcooled fluid, consisting of molecules, its absorption occurs in the 1st and 2nd ways, but because of the matter it is composed of, it absorbs outside our visible spectrum.
Essentially because of absorption. When photon flies into the material it interacts with its constituents. This interaction can be divided into two contributions. One of them is elastic and is the source of the index of refraction because effectively it just slows the photon down while the other one is inelastic. Photon gets absorbed by an atom say and later it is emitted as thermal radiation in random direction thereby losing the original information it carried.
When you look at this macroscopically, this process will be described by some parameter like penetration depth and intensity w. So if you made opaque objects thin enough, they would still be transparent although the outgoing light would be weaker depending on thickness. Of course, this discussion completely avoids surface effects reflection, refraction, scattering, etc. Note that all of this depends on frequency of the incident light. Atoms let's just talk about them for simplicity; in reality there will be contribution also from molecules, lattice, free electrons and whatnot have something called absorption spectrum.
This arises because for certain frequencies electron can catch the photon and get excited to the higher energy state. So, while a material can be transparent in certain range of frequencies like glass is for visible light it can be quite opaque in others. This may be a little technical, but I always thought it was cool: one of my professors once pointed out that transparency only happens because the material is approximately a linear dialectic over the frequency range that you care about.
A solid material with the chaotic structure of a liquid, which allows the molecules to freely fill in any gaps. This makes the surface of glass uniform on a microscopic level, allowing light to strike it without being scattered in different directions. But this still doesn't explain why light is able to pass through glass rather than being absorbed as with most solids. For that, we need to go all the way down to the subatomic level.
You may know that an atom consists of a nucleus with electrons orbiting around it, but you may be surprised to know that it's mostly empty space. In fact, if an atom were the size of a sports stadium, the nucleus would be like a single pea in the center, while the electrons would be like grains of sand in the outer seats. That should leave plenty of space for light to pass through without hitting any of these particles. So the real question is not why is glass transparent, but why aren't all materials transparent?
The answer has to do with the different energy levels that electrons in an atom can have. Think of these as different rows of seats in the stadium stands.
An electron is initially assigned to sit in a certain row, but it could jump to a better row, if it only had the energy. As luck would have it, absorbing one of those light photons passing through the atom can provide just the energy the electron needs. Like this: Like Loading Related Posts. Tags physics who what why when. Search Search for:. Local Weather Guernsey. Fri Sat Sun. Guernsey Airport Arrivals Departures. Recent Random. What would have happened if the gunpowder plot had succeeded?
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