What does it take for a material to be transparent to light?

[refind]

I need help exploring this question: what makes a solid transparent/opaque?
If I have clear sheet of plastic and I introduce imperfections (such as small air bubbles inside), it turns white if the # of imperfections is large. Is it possible to maintain the polymer transparent with such air pockets inside? If bubbles are made very small (say taking the limit as diameter -> 0), would that do the trick?

Any input appreciated.

 

[davenn]

firstly you need to define what frequency of light you are interested in.
materials opaque or near opaque to visible light may well be transparent to IR or UV light
or any combination of those

Maybe you are just asking a generalised question ?


Dave

 

[chill_factor]

any material is transparent if the frequency is high enough.

For visible and below, a nanoscale uniform dielectric like glass, monocrystalline aluminum oxide or many thermoplastic polymers are pretty good.

Porous materials are not transparent due to microscale scattering centers like air pores.

 

[refind]

I'm interested in visible light, so frequency is constrained. What would it take to make a thermoplastic polymer with impurities be transparent to visible light frequencies? If I can make the air bubbles very small (few nm), would that do the trick regardless of impurity shape and density (number of bubbles per unit volume).
Are there any theories/papers on this subject that may be helpful?

Thanks!

 

[chill_factor]

I'm interested in visible light, so frequency is constrained. What would it take to make a thermoplastic polymer with impurities be transparent to visible light frequencies? If I can make the air bubbles very small (few nm), would that do the trick regardless of impurity shape and density (number of bubbles per unit volume).
Are there any theories/papers on this subject that may be helpful?

Thanks!

try plastic wrap for food.

 

[jaydnul]

any material is transparent if the frequency is high enough.

What? If this were true, what would be the point of sunscreen? If anything, it would be lower frequencies that are more transparent, at least from what I've seen. Would I need to worry about gamma rays from a nearby supernova?

 

[Curl]

try plastic wrap for food.

what do you mean "try" it?

 

[chill_factor]

What? If this were true, what would be the point of sunscreen? If anything, it would be lower frequencies that are more transparent, at least from what I've seen.

Depends on what material. Lower frequencies are more transparent to dielectrics but have no hope of going through conductors. When I say low frequencies I mean frequencies below the plasmon frequency. For frequencies above the plasmon frequency the metal is transparent.

Sunscreen is special in that it has molecular transitions in the near UV energy range that gets through to earth.

 

[chill_factor]

what do you mean "try" it?

plastic food wrap is transparent. it is also a thermoplastic.

 

[ehild]

A porous but not absorbing material appears white as the light scatters on the pores. That can be reflection at the interface of the bubble and the matrix, or diffraction on the small imperfection. The rays coming from a light source will not travel in the original direction, you can not see it through the sheet of material. The scattering depends on the relative size of the pores with respect to the wavelength. Bubbles, size of a few nanometer do not scatter visible light appreciably. Scattering also depends on the refractive indices of the material filling the bubbles with respect to the matrix itself. White paper is not transparent, but transparency improves if you soak it in oil.

If you want a professional article about porous plastics, see http://www.lbl.gov/Tech-Transfer/publications/2519pub.pdf for example.

ehild

 

[mfb]

What? If this were true, what would be the point of sunscreen?

"High enough" is in the range of x-rays and gamma rays. You need a lot of material to block those. They are not an issue in the solar radiation, however, and the atmosphere can block most of them.

Would I need to worry about gamma rays from a nearby supernova?

Only if it is very close to us.