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SuperJoost23

Why doesn't UV go through normal glass

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I have a big assignment for school, in which I will be researching wether or not it is dangerous or not to wear cheap sunglasses, as you have your eyes more open when there is less visible light, if UV was to come through cheap glasses it could be dangerous.

Now I do understand why some wavelengths of UV have a higher penetration power, however I don't understand what defines what material is good at letting UV through or blocking it. Is it just density? Does is have something to do with the way molecules are arranged? How could I find what is a good UV blocker, and why it is?

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It's whether or not there are transitions in the material that will absorb the photons of a particular wavelength. That depends on the particular molecules that comprise the material.

 

 

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40 minutes ago, SuperJoost23 said:

I have a big assignment for school, in which I will be researching wether or not it is dangerous or not to wear cheap sunglasses, as you have your eyes more open when there is less visible light, if UV was to come through cheap glasses it could be dangerous.

It's not cheap that matters, it's the  lack of side protection/blocking that is the issue with dilated pupils in subdued light.  Because the acceptance angle for incoming light increases with wider pupils, more uv light can come in from the side and bounce off the inside glass surface into the eye. Wraparound glasses are best in this respect for blocking off this sidelight. Apparently, wearing sunglasses without side blocking is worse than wearing no sunglasses at all.

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Now I do understand why some wavelengths of UV have a higher penetration power, however I don't understand what defines what material is good at letting UV through or blocking it. Is it just density? Does is have something to do with the way molecules are arranged? How could I find what is a good UV blocker, and why it is?

UV has a higher energy, not penetrating power. It is able to excite the outer electrons of atoms better than other wavelengths. If a photon is the right wavelength to interact with an electron it will be absorbed...its path will be blocked. If the photon is the 'wrong' wavelength it will briefly interact with an electron but will immediately get ejected from the electron on the same path it came at it.. this is what makes glass appear transparent. What we want  a sunglass material to do is absorb the UV and transmit the other colours to the eye.  Check out the absorption properties of glass and polycarbonate and find out what they block.

Edited by StringJunky

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From my notes on Introductory/conceptual physics (IE not at all advanced)

Transparent materials: the electrons stay in their orbitals. Oscilating electric field of incident light (the light that hits the electrons) causes a temporary vibration of the electron cloud - withOUT the electrons changing to a higher energy orbit. They pass the vibration through. When it gets to the end of the material the final electrons pass on the vibrations by re-emitting the light wave. The effect is that light has passed through the transparent material at a slower speed than it would in a vacuum (or in the atmosphere).

This only happens when the material's electrons match correctly the frequency of the E-M wave. I glass this is visible light.

Opaque material: The material absorbs the E-M wave, the whole molecule vibrates not just the electron. The energy is absorbed (get's warmer). The light wave is not transmitted through the material but is absorbed.

Glass is a transparent material to visible light, it is opaque to UV light.

This does not answer your question the way that StringJunky does, but gives some background.

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On 1/7/2019 at 5:52 PM, StringJunky said:

Because the acceptance angle for incoming light increases with wider pupils, more uv light can come in from the side and bounce off the inside glass surface into the eye.

If the glass is a good absorber of UV- which it should be- it won't reflect the UV. Even if it was a non- absorber like fused quartz, the reflections would only be something like 10% as intense as the direct light.

The dominant UV absorber in most glass is usually iron, preset as an impurity from the sand used in glassmaking.

For plastic glasses it's likely that much of the UV is absorbed by the polymer. 

 

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17 minutes ago, John Cuthber said:

If the glass is a good absorber of UV- which it should be- it won't reflect the UV. Even if it was a non- absorber like fused quartz, the reflections would only be something like 10% as intense as the direct light.

The dominant UV absorber in most glass is usually iron, preset as an impurity from the sand used in glassmaking.

For plastic glasses it's likely that much of the UV is absorbed by the polymer. 

 

That was a bit I picked up off an OU programme a long time ago.  

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On 1/10/2019 at 3:51 AM, StringJunky said:

That was a bit I picked up off an OU programme a long time ago.  

Many years ago, as a kid, we heard about the "goodness" present in Sunlight, that is, UV. UV lamps were sold specifically to be placed in the rooms of infants to "cleanse" the air. Window glass was marketed with the quality of transmitting far more UV than normal glass: it was called "Vitaglas", possibly with two "s"es. I have searched in vain for reference to Vitaglas.

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1 minute ago, tinkerer said:

UV lamps were sold specifically to be placed in the rooms of infants to "cleanse" the air.

UV photons are killing microbes.

"Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses short-wavelength ultraviolet (UV-C) light to kill or inactivate microorganisms by destroying nucleic acids and disrupting their DNA, leaving them unable to perform vital cellular functions.[1] UVGI is used in a variety of applications, such as food, air, and water purification."

https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation

Kids should not be present in the room when it's turned on..

1 minute ago, tinkerer said:

Many years ago, as a kid, we heard about the "goodness" present in Sunlight, that is, UV. UV lamps were sold specifically to be placed in the rooms of infants to "cleanse" the air. Window glass was marketed with the quality of transmitting far more UV than normal glass: it was called "Vitaglas", possibly with two "s"es. I have searched in vain for reference to Vitaglas. 

That's because UV light present in the Sunlight stimulates production of Vitamin D in the skin.

"The major natural source of the vitamin is synthesis of cholecalciferol in the skin from cholesterol through a chemical reaction that is dependent on sun exposure (specifically UVB radiation)."

https://en.wikipedia.org/wiki/Vitamin_D

 

 

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11 minutes ago, tinkerer said:

Many years ago, as a kid, we heard about the "goodness" present in Sunlight, that is, UV. UV lamps were sold specifically to be placed in the rooms of infants to "cleanse" the air. Window glass was marketed with the quality of transmitting far more UV than normal glass: it was called "Vitaglas", possibly with two "s"es. I have searched in vain for reference to Vitaglas.

Yeah, when Coca Cola was the real thing. :)

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9 hours ago, tinkerer said:

Many years ago, as a kid, we heard about the "goodness" present in Sunlight, that is, UV. UV lamps were sold specifically to be placed in the rooms of infants to "cleanse" the air. Window glass was marketed with the quality of transmitting far more UV than normal glass: it was called "Vitaglas", possibly with two "s"es. I have searched in vain for reference to Vitaglas.

Radium was once marketed in a similar fashion.

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