Does Glass Absorb Light in the Visible Spectrum?

[pinky]

hey guys i found this little bit of info about glass and i was wondering if it is true or not.

the statement was:

the energy levels of the individual atoms in glass are not such that they absorb light in the visible spectrum, although they will absorb infrared and ultraviolet;

So is this true? and if it is can some one please explain it to me.

Thanks allot

Pinky

 

[Njorl]

The ultraviolet is absorbed. I believe you have to go pretty far into the infrared to get significant absorption.

I'm not sure about the mechanism of absorption. I believe, for the ultraviolet, it has something to do with the vitreous nature of glass. Quartz, which is just a crystalline form of typical glass, is transparent to much of the ultraviolet spectrum.

Njorl

 

[pallidin]

If I remember correctly, a heat lamp has a covering of quartz "glass" due to this very reason.

 

[pinky]

thanks guys, i really never thought of glass being able to block UV and IR light.
any more info about how IR light gets through glass?

Pinky

 

[selfAdjoint]

UV ought to be an easy conclusion. You can't get a tan through a window. By the same reasoning I am doubtful about IR blocking; you can get warmed through a window. Ever got into a car parked outside for a while on a sunny cold day?

 

[GCT]

Light is not reflected nor absorbed for transparent glass, maybe refracted.

 

[nautica]

Originally posted by GeneralChemTutor
Light is not reflected nor absorbed for transparent glass, maybe refracted.

refraction is due to absorption.

Nautica

 

[pallidin]

Originally posted by selfAdjoint
UV ought to be an easy conclusion. You can't get a tan through a window. By the same reasoning I am doubtful about IR blocking; you can get warmed through a window. Ever got into a car parked outside for a while on a sunny cold day?

I may well stand corrected. I remember the part about quartz "glass" being used for certain types of lighting, but I think it was with respect to lamps used for tanning(UV), not infrared as I posted.

 

[nautica]

Originally posted by pallidin
I may well stand corrected. I remember the part about quartz "glass" being used for certain types of lighting, but I think it was with respect to lamps used for tanning(UV), not infrared as I posted.

So is it the quarts that blocks the UVA and let's the UVB through in the more expensive beds??

Nautica

 

[Phantom]

Originally posted by nautica
refraction is due to absorption.

Nautica

Refraction and absorption are two separate light transformations-

absorption is a chemical effect, ie, interaction with dyes or pigments
refraction is purely a physical effect - dependent on the differential of densities in the glass or other media

The two are not causally related.

 

[nautica]

At a molecular level the material will absorb the light particle and then release it. This is what causes the refraction. Which is the same thing as slowing of light. (Or at least the apparent slowing)

Nautica

 

[gradies]

By the same reasoning I am doubtful about IR blocking; you can get warmed through a window. Ever got into a car parked outside for a while on a sunny cold day?

Any frequency of light warms a surface that it is absorbed by, not just those sitting within the IR range, so the heat your feeling could easily be from visible light which corresponds to a higher temperature than IR. Thermal radiation being confined to the IR region is a common misconception because that is the frequency of light emitted by human beings. However, as an object increases in temperature the radiation being given increases in frequency. Then once that frequency moves from the IR range to the visible we begin to observe the radiating photons. This is why hot metal glows "red hot" and fire is simply gas that is hot enough for us to see the radiating photons. Red being the first frequency we can see and then moving up through the rainbow.

UV/IR absorption/reflection is very real and this is why glass prescription glasses shield your eyes from UV. Also a friend of mine is currently conducting an experiment in which he needs to observe mice in a glass cage in the dark. In order to use a night vision camera which uses infrared reflection he needed to place the source of the infrared light within the cage because when it was outside the glass none of the light got through.

Lastly Phantom is right. Refraction is when light slows down and increases in frequency while inside a medium causing the lights path to bend when entering and exiting the medium. Absorption is when an electron steals the energy within light, an electromagnetic wave, in order to increase it’s “energy level.” Thus increasing the temperature of the object.

 

[Nereid]

Welcome to Physics Forums gradies!

IIRC there was an exciting development in the movie industry a few decades back - a 'transparent' filter for those very bright lights they needed then to do their shooting. It screened out the IR (or so the PR said), and they could shoot stars eating ice cream without it melting everywhere (and without needing to add funny chemicals to stop the icecream melting).

I wonder how hot the filter got?

 

[gradies]

Thanks for the welcome.

Well, since all electromagnetic radiation transfers heat, by cutting out any/all the radiation transmitted from a white light source that isn’t visible you would drastically decrease the radiated heat while not loosing any of the light necessary to maintain light levels for the camera. Like you said though, those filters must have gotten damn hot.

Light and convection are the only ways that heat can travel through air, so if you don’t feel a draft, but your getting warmer, then your absorbing light.

Nautica: if refraction is just a quick absorption and reemission then why is there a decrease in the wavelength of the light while it is traveling through the medium?

 

[nautica]

that would be the reason for the decrease in wave length. Right?

Nautica

 

[gradies]

that would be the reason for the decrease in wave length. Right?

explain

 

[pinky]

i know this is wondering a little off my first post but can any of you help me with the strength of glass, i can't seem to find much about it . any info is great,
BTW on my original topic. you guys are really helping out.

Pinky

 

[Phantom]

Originally posted by gradies
Thanks for the welcome.

Well, since all electromagnetic radiation transfers heat, by cutting out any/all the radiation transmitted from a white light source that isn’t visible you would drastically decrease the radiated heat while not loosing any of the light necessary to maintain light levels for the camera. Like you said though, those filters must have gotten damn hot.

Light and convection are the only ways that heat can travel through air, so if you don’t feel a draft, but your getting warmer, then your absorbing light.

Nautica: if refraction is just a quick absorption and reemission then why is there a decrease in the wavelength of the light while it is traveling through the medium?

The propagation of light through any medium is dependent on the properties of the medium, in this case, glass, where refraction is caused by a relative change in density. Since refraction always results in a change in wave speed, hence there is an accompanying change in wavelength.

 

[gradies]

Since refraction always results in a change in wave speed, hence there is an accompanying change in wavelength.

please don't take offence, but these are assertions not explanations. I understand how in order for light to slow down it's wavelegth must decrease. What we were discussing is how the absorption/reemission explanation of refraction accounts for the wavelength decrease. By the way I'm visualizing that process. It would take longer for the light to get through the medium because it would spend time not existing, but whenever it did exist it seems as though would have the same wavelength.

So either the wavelength doesn't decrease. But I believe that's been proven.

I'm visualizing this theory wrong. Very likely.

Or this theory is bogus. I'd bet on this one.

 

[Phantom]

That WAS the explanation. Maybe I can make it a little more clear. Specifically, why refraction results in a change in wave speed, which I explained, is due to a density differential. When passing thru glass for example which goes from a less dense to more dense material, or vice versa. The denser material has a higher coefficient of absorption and consequently emits light at a lower wavelength than the incoming wave.

Yes, there is always some scattering, both inelastic and elastic depending on the material and original wavelength. This is why you have envisioned some period of time in which the wave does not exist. Its not that it doesn't exist, but that its being scattered by the refracting material. And hence you are correct, it does take longer to pass through refractive material than through a vaccuum. That is fundamentally correct!

BTW, this is not at all a bogus theory. Boundary physics in QED has several proven shifts for standard light transformations, like the Stokes shift, which explains luminescence, for example.

Hope that helps.

 

[Njorl]

Originally posted by mmwave
One reason a car in the sun gets so much hotter than the outside air temp. in the sun (thereby killing helpless critters inside like babies or dogs) is that the UV comes in, gets absorbed in the furnnishings and it re-emitted as IR that cannot pass back out through the window glass. Thus the heat is trapped inside the car making its temp. much higher than the outside air temp.

It is the visible that gets in, not the UV. The peak intensity of terrestrial sunlight is in the visible. While UV may be more energetic, the low intensity makes it rather unimportant for heating purposes.

Njorl

 

[davilla]

Okay, so I believe glass absorbs IR from the heated car example. But at the same time glass makes a pretty poor insulator, as you'd discover next to a single-paned window in the dead of winter. Is that because glass is at least somewhat transparent in the IR, or is there something else at work here?

BTW, I've heard a lightbulb described as a radiator with the side effect of light production.

 

[Phantom]

Glass is a poor insulator generally without coatings or glazings. The R value of a single pane of glass is about 1. But a car is somewhat of a poor example of this because most of the energy aborbed by a car is transmitted via conduction and convection to the interior.