1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is it possible to have perfect white or mirrors?

  1. Aug 31, 2012 #1
    Is it possible for a surface that reflects all visible light, and absorbs absolutely none to exist, Or would that violate the second law of thermodynamics?
     
  2. jcsd
  3. Aug 31, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I think it's more that the color of a surface is more complicated than the simple model you are using will allow for.

    But you will always get some absorbtion of some wavelengths.
    To get something very white - look at TiO2 and ZnO spectra.
     
  4. Aug 31, 2012 #3

    phinds

    User Avatar
    Gold Member
    2016 Award

    Visible light, while only a small part of the electromagnetic spectrum, is still quite a range so I don't see how you could EVER have a material that could reflect all the frequencies perfectly. I think I'm saying the same thing Simon said.

    As for a perfect mirror ... good question. I THINK I remember reading that there is some reason it's impossible for any mirror to be perfect, but I don't recall what the rationale for that statement was, and don't know if it's true.
     
  5. Aug 31, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    iirc: classically you can get perfect reflection for one wavelength by interference methods - and, then, there is total internal reflection if we call the inner surface a "mirror".

    On the scale of photons - all bets are off: there is no such thing as a "surface" the way we are used to thinking of it. See the Feynman QED descriptions of reflection ... you can increase the total reflection from a surface by removing more than half of it.
     
  6. Aug 31, 2012 #5

    jbriggs444

    User Avatar
    Science Advisor

    You could argue that an otherwise perfect mirror would have to recoil under the momentum of the light that it is reflecting and that this would red-shift the reflected light.
     
  7. Aug 31, 2012 #6

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    Of course not- you also can't fabricate a perfectly flat surface or a frictionless surface. Some real materials (for example, Spectralon and InfraGold) provide >99% reflectivity over a reasonably broad wavelength range:

    http://www.pro-lite.co.uk/File/spectralon_material.php

    http://www.labsphere.com/products/reflectance-materials-and-coatings/infragold.aspx [Broken]
     
    Last edited by a moderator: May 6, 2017
  8. Sep 1, 2012 #7
    Can a black hole send a beam of light back to its source if the beam comes in at the right angle? Without changing the light?
     
  9. Sep 1, 2012 #8

    K^2

    User Avatar
    Science Advisor

    Sure, but from an infinitely narrow crossection, so any real light beam will get at least somewhat scattered.
     
  10. Sep 1, 2012 #9

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Superconductors are excellent reflectors for low-frequency photons. But even there, you have some experimental limits - the superconductor material has some border, it might have small hot areas which are not superconducting, and so on.
     
  11. Sep 1, 2012 #10
    You specified reflectivity at visible wavelengths. However, your question about thermodynamics implies broader conditions. So I think you were asking if it was possible for a mirror that reflects all the energy from electromagnetic waves completely. I don’t think there are any theoretical limits for such reflectivity restricted to the visible wavelengths. However, I would need to examine the Kramers-Kronig relationship more closely to determine if what you say is true restricted to the visible wavelengths.
    The existence of a mirror that reflects 100% of the electromagnetic wave energy from its surface does not contradict any law in thermodynamics. Or at least I haven’t seen any such theorem which says a mirror that completely reflects violates thermodynamics.
    However, the existence of such a mirror probably contradicts the Kramers-Kronig relations. The Kramers-Kronig relations relate the spectrum of the index of refraction to the spectrum of the absorption.
    According to the Kramers-Kronig relationships, the reflectivity is high when the absorptivity is strong. The converse is not always true. However, there is always a strong absorption somewhere in the frequency spectrum when there is a high reflectivity somewhere in the reflectivity spectrum. You can’t have a high reflectivity without a strong absorption, somewhere.
    The indices of refraction of two media at a surface between them determine the reflectivity. In order to reflect 100% over any spectral range, the index of refraction of one of the media has to be infinite. When you apply the Kramers-Kronig relations to such high index of refraction, one would get rather high oscillations in absorption at high frequencies. A high index of refraction over a spectral range implies that the material has a high absorptivity toward the higher frequencies of this range.
    I suspect that you are intuiting this type of relationship. So you should look into the Kramers-Kronig relations to see if you are right.
    The Kramers-Kronig relations have a physical basis in a property called “macroscopic causality”. However, here are links to introduce you to Kramers-Kronig relations.


    http://en.wikipedia.org/wiki/Kramers–Kronig_relations
    “The Kramers–Kronig relations are bidirectional mathematical relations, connecting the real and imaginary parts of any complex function that is analytic in the upper half-plane. These relations are often used to calculate the real part from the imaginary part (or vice versa) of response functions in physical systems because causality implies the analyticity condition is satisfied, and conversely, analyticity implies causality of the corresponding physical system.[1] The relation is named in honor of Ralph Kronig[2] and Hendrik Anthony Kramers.”

    http://media.wiley.com/product_data/excerpt/26/04700219/0470021926.pdf
    “thus. the reflectance is substantial while absorption is strong.”

    http://en.wikipedia.org/wiki/Refractive_index
    “The real and imaginary parts of the complex refractive index are related through the Kramers–Kronig relations. For example, one can determine a material's full complex refractive index as a function of wavelength from an absorption spectrum of the material.”
     
  12. Sep 2, 2012 #11
    I just read an article about "Total internal reflection", and it said something about a light being 100% reflected if it strikes a surface at an angle smaller than the "critical angle". Am I understanding this correctly?
     
  13. Sep 3, 2012 #12

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    That is correct ... that is what Snell's Law predicts.
    However: Snell's Law is only obeyed on average.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook