Why is Glass Opaque to IR and UV, But Not Visible Light?

  • Thread starter peter.ell
  • Start date
  • #1
43
0
According to my understand of what makes something transparent, it's because a given wavelength of light doesn't have sufficient energy to raise the electrons in a material to a higher energy level, so the photons are not absorbed and continue on.

This explains why glass is transparent for visible light but absorbs UV, as UV has enough energy to excite the electrons to the allowed energy levels of silicon...but then why is glass also opaque to lower energy IR?

Thank you.
 

Answers and Replies

  • #2
phinds
Science Advisor
Insights Author
Gold Member
2021 Award
17,358
8,886
Hm ... if glass is opaque to UV and IR, I wonder why it is that my car seat gets so damned HOT in the summer?
 
  • #3
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,073
Don't solar cells (silicon) use mostly visible light? So there must be a fair amount of energy in the visible spectrum of sunlight.
 
  • #4
43
0
Hm ... if glass is opaque to UV and IR, I wonder why it is that my car seat gets so damned HOT in the summer?

It's actually because glass is transparent to visible light, which is absorbed by the materials inside your car and then that light is re-radiated at a longer wavelength--IR. This IR is then trapped inside of the car because the glass windows keep it in.
 
  • #5
davenn
Science Advisor
Gold Member
2021 Award
9,582
8,651
It's actually because glass is transparent to visible light, which is absorbed by the materials inside your car and then that light is re-radiated at a longer wavelength--IR. This IR is then trapped inside of the car because the glass windows keep it in.

ahhh but the flaw in your logic is that the glass window let the IR in for a start

Glass doesnt stop UV or IR ... if it did IR or UV cameras would not work they ALL have glass lenses
the only time glass WONT pass either IR or UV is if it is specially treated to produce a filter to those wavelengths. for example I have a screw on UV filter on my digital SLR camera
I wouldnt need that if glass normally stopped UV

cheers
Dave
 
Last edited:
  • #6
davenn
Science Advisor
Gold Member
2021 Award
9,582
8,651
According to my understand of what makes something transparent, it's because a given wavelength of light doesn't have sufficient energy to raise the electrons in a material to a higher energy level, so the photons are not absorbed and continue on.

Thank you.

actually looking at the first part of your post, that isnt quite right. According to the late Prof. Richard Feynman, basically, photons entering the glass are absorbed by the atoms of the glass exciting electrons causing those electrons to emit new photons that carry on through the glass.
that is, I understood from his lectures, that the photons coming out the other side of the glass (or those that appear to be reflected out the same side) are not necessarily the same ones that went into the glass.

cheers
Dave
 
Last edited:
  • #7
davenn
Science Advisor
Gold Member
2021 Award
9,582
8,651
It's actually because glass is transparent to visible light, which is absorbed by the materials inside your car and then that light is re-radiated at a longer wavelength--IR. This IR is then trapped inside of the car because the glass windows keep it in.

Another flaw in your logic ...... ;)

in a ventilated area, put your arm under a sheet of glass in summer sun, I bet it gets seriously sunburnt. The IR energy isnt getting trapped anywhere except in your skin!! :)

Not sure where your got your understandings from, but unfortunately, you were seriously misled

cheers
Dave
 
  • #8
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,073
ahhh but the flaw in your logic is that the glass window let the IR in for a start

He didn't say that it does. In fact, he states that it doesn't.

Glass doesnt stop UV or IR ... if it did IR or UV cameras would not work
Wrong.
 
  • #9
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,073
put your arm under a sheet of glass in summer sun, I bet it gets seriously sunburnt. The IR energy isnt getting trapped anywhere except in your skin!! :)

IR? Sunburn? Really?

Another flaw in your logic ...... ;)

Not sure where your got your understandings of sunburn from, but unfortunately, you were seriously misled
 
  • #10
davenn
Science Advisor
Gold Member
2021 Award
9,582
8,651
He didn't say that it does. In fact, he states that it doesn't.

yes he does read his original comments!! he states IR and UV doesnt pass through glass


Wrong.

try again as I said if glass stopped UV then there would be no need for me to use UV filters!!!

I really dont know if you are just picking a fight/argument
or you really have no understanding of optics

I suspect the former and you are just wanting to be nasty
in which case I will leave you to your stupidity as I have no inclination of trying to deal with the likes of you any further
 
  • #11
wiki:
Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.
http://en.wikipedia.org/wiki/Ultraviolet
UV is from 10 to 400 nm, so no, you won't get a sunburn trough the glass.

Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. In addition, the warmed structures and plants inside the greenhouse re-radiate some of their thermal energy in the infra-red, to which glass is partly opaque, so some of this energy is also trapped inside the glasshouse.
http://en.wikipedia.org/wiki/Greenhouse
 
  • #12
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,073
yes he does read his original comments!! he states IR and UV doesnt pass through glass
That's what I said he said.

try again as I said if glass stopped UV then there would be no need for me to use UV filters!!!
If glass didn't heavily attenuate UV then there would be no need for vets to warn pet owners that sitting bird and reptile cages beside a window will not provide their pets with adequate daily UV light unless the the window is actually open.

I really dont know if you are just picking a fight/argument
or you really have no understanding of optics
Just gently correcting your misunderstanding, actually. I think where you're going wrong is with your perception of attenuation as an "all or nothing" thing. It isn't. While all frequencies of light are transmitted through ordinary glass, UV is more heavily attenuated than is visible light.

<rant elided>
 
  • #13
sophiecentaur
Science Advisor
Gold Member
26,980
5,783
actually looking at the first part of your post, that isnt quite right. According to the late Prof. Richard Feynman, basically, photons entering the glass are absorbed by the atoms of the glass exciting electrons causing those electrons to emit new photons that carry on through the glass.
that is, I understood from his lectures, that the photons coming out the other side of the glass (or those that appear to be reflected out the same side) are not necessarily the same ones that went into the glass.

cheers
Dave

I don't think that can be quite right - at least not when it's put in that blunt way. If new photons were generated as a result of old ones being absorbed, the re-radiated em waves would be in all directions so no coherent image could be seen. ( You couldn't identify actual 'Rays' going through.) The only time that photons are generated in this, coherent, way is during Laser operation and that only happens when Stimulated Emission takes place.

It is true to say, however, that photons do react with the medium they pass through, though (the velocity of the wave slows down) so there is 'something' in what you say - it's not just that simple, I think.

Transmission of em through materials is different for all frequencies (energies). Gamma rays tend to get through any but the most dense metals fairly easily - they just hit the nuclei and may get absorbed. Low energy (RF) waves do not 'pass through' metals but generate currents by generally 'vibrating' all the free electrons in the metal (in a 'bulk' interaction - rather than a one photon / one electron interaction. This causes reflection plus some absorption.

For crystals and amorphous (covalent?) materials, (insulators, at least), there can be slight shifting (polarising) of the charges within molecules which may involve more or less loss / absorption, depending on the wavelength and the material. Perhaps this is somewhere near what you got from the Feynman lecture? He did love his Feynman Diagrams but he stressed that they are only meant to be a short hand version of what goes on and shouldn't be regarded as an rigorous description of a 'mechanism'.
 

Related Threads on Why is Glass Opaque to IR and UV, But Not Visible Light?

Replies
7
Views
3K
Replies
6
Views
24K
Replies
3
Views
1K
Q
  • Last Post
Replies
8
Views
2K
Replies
3
Views
3K
Replies
8
Views
3K
  • Last Post
Replies
10
Views
7K
  • Last Post
Replies
2
Views
16K
Top