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Re >> "with an ability to reflect around 98% of incoming sunlight" Seems like a mirror would reflect even more light. Now, obviously we don't want to make the

Re >> "with an ability to reflect around 98% of incoming sunlight" Seems like a mirror would reflect even more light. Now, obviously we don't want to make the exteriors of our buildings and cars a mirror!. but it made me wonder: What's the difference between a really really really white paint that reflects 98%+ light, vs. a reflective surface? Is it that the white paint reflects the light, but in a "scrambled" form, thus losing the image?

wffurr 4y ago
Yes, it's not reflecting the light so much as re-radiating it in a diffuse pattern.
- thehappypm 4y ago
  I don’t think there’s a physical difference between reflection and re-radiation, is there? Reflection is just highly organized/non-diffuse.
  - Fronzie 4y ago
    reflection is coherent, a single physical process. The light keeps it's phase and polarization. Re-radiation is incoherent, caused by multiple particle interactions (physical processes). So there's quite a difference in physics.
    - thehappypm 4y ago
      Can you explain? As I understand it, in both cases, a photon comes in, gets absorbed by the material, and then gets re emitted.
njdvndsjkvn 4y ago
yes. A white surface scatters the photons and a reflective one keeps them in the same position relative to the others. If you make a surface smooth enough it starts becoming partly reflective. There is also scattering of photons that happens in object itself I found this video about the topic to be interesting: https://www.youtube.com/watch?v=1n_otIs6z6E https://www.youtube.com/watch?v=1n_otIs6z6E (title: Why aren't Mirrors White? Why isn't EVERYTHING a Mirror?)
adrian_b 4y ago
Nope. Silver reflectivity is about 95%, aluminum reflectivity is about 90%. So the best metallic mirrors are worse than the best white paints, even if they can be better than the frequently used white paints based on titanium white (89.5% reflectivity). Total internal reflection might be closer to 100%, but that cannot be used for the light that comes from the air on the surface of a mirror.
- njdvndsjkvn 4y ago
  A simple way to verify this is to observe that common mirrors also tend to be green, showing that the other colors are absorbed more by the mirror. If you face two mirrors towards each other you will see a lot of green. edit: here's an example: https://i.imgur.com/Ar80FYZ.jpeg https://i.imgur.com/Ar80FYZ.jpeg
  - adrian_b 4y ago
    The common mirrors have the metal on the back side, so their reflectivity is diminished by the absorption in the glass. The better mirrors, which are used e.g. in telescopes and some other optical instruments, have the metal on the front side, but that limits the metal choice to aluminum, because silver is tarnished quickly by the sulfur compounds present in the air.
    - njdvndsjkvn 4y ago
      so why don't common mirrors just use aluminum? It's already cheaper than silver, right?
      - adrian_b 4y ago
        The common mirrors have been using aluminum for a long time. I do not know when aluminum has replaced silver, but it was probably not a long time after WW2. The silver mirrors had the advantage that the metal could be easily deposited from a water solution, anyone could make silver mirrors at home, if desired. The aluminum mirrors use a vacuum deposition process, which requires very expensive equipment, but the cost per mirror is much less.
GuB-42 4y ago
To make things simple, yes, that's is. White "scrambles" the light, the correct term is "diffuses" but that's the idea. Maybe quantum physicists will disagree but the difference between a mirror and a white surface is that a mirror is perfectly flat and therefore will reflect light at always the same angle. It may absorb some of it, or even most of it (black mirror!) but what it reflects, it will do it always the same way. A white painted surface can be thought as a surface covered in tiny mirrors, all pointed in a different direction, and depending on which tiny mirror a ray of light will hit, it will be reflected in a different direction. With large pieces, like with a broken mirror, you will still see (as you said) scrambled images. But because the pieces are so tiny, you won't be able to make out an image and you will only see white. A glass pane can look like a mirror, but reduce it to a powder and it becomes white. It has nothing to do with absorption, you can have a 1% reflective mirror, and 100% reflective diffuse white. The way the "tiny mirrors" that make up the white paint are oriented give out its general look. Have them oriented mostly with the surface and you get a glossy finish, have them randomly oriented in many directions and you get a matte finish (this is called specular reflection btw). I've seen mirrors be called "smart white", it is white because it reflects most light in most wavelengths, and it is smart because it keeps the rays of light oriented.

wffurr 4y ago

Yes, it's not reflecting the light so much as re-radiating it in a diffuse pattern.

thehappypm 4y ago
I don’t think there’s a physical difference between reflection and re-radiation, is there? Reflection is just highly organized/non-diffuse.
- Fronzie 4y ago
  reflection is coherent, a single physical process. The light keeps it's phase and polarization. Re-radiation is incoherent, caused by multiple particle interactions (physical processes). So there's quite a difference in physics.
  - thehappypm 4y ago
    Can you explain? As I understand it, in both cases, a photon comes in, gets absorbed by the material, and then gets re emitted.

njdvndsjkvn 4y ago

yes. A white surface scatters the photons and a reflective one keeps them in the same position relative to the others. If you make a surface smooth enough it starts becoming partly reflective. There is also scattering of photons that happens in object itself I found this video about the topic to be interesting: https://www.youtube.com/watch?v=1n_otIs6z6E https://www.youtube.com/watch?v=1n_otIs6z6E (title: Why aren't Mirrors White? Why isn't EVERYTHING a Mirror?)

adrian_b 4y ago

Nope. Silver reflectivity is about 95%, aluminum reflectivity is about 90%. So the best metallic mirrors are worse than the best white paints, even if they can be better than the frequently used white paints based on titanium white (89.5% reflectivity). Total internal reflection might be closer to 100%, but that cannot be used for the light that comes from the air on the surface of a mirror.

njdvndsjkvn 4y ago
A simple way to verify this is to observe that common mirrors also tend to be green, showing that the other colors are absorbed more by the mirror. If you face two mirrors towards each other you will see a lot of green. edit: here's an example: https://i.imgur.com/Ar80FYZ.jpeg https://i.imgur.com/Ar80FYZ.jpeg
- adrian_b 4y ago
  The common mirrors have the metal on the back side, so their reflectivity is diminished by the absorption in the glass. The better mirrors, which are used e.g. in telescopes and some other optical instruments, have the metal on the front side, but that limits the metal choice to aluminum, because silver is tarnished quickly by the sulfur compounds present in the air.
  - njdvndsjkvn 4y ago
    so why don't common mirrors just use aluminum? It's already cheaper than silver, right?
    - adrian_b 4y ago
      The common mirrors have been using aluminum for a long time. I do not know when aluminum has replaced silver, but it was probably not a long time after WW2. The silver mirrors had the advantage that the metal could be easily deposited from a water solution, anyone could make silver mirrors at home, if desired. The aluminum mirrors use a vacuum deposition process, which requires very expensive equipment, but the cost per mirror is much less.

GuB-42 4y ago

To make things simple, yes, that's is. White "scrambles" the light, the correct term is "diffuses" but that's the idea. Maybe quantum physicists will disagree but the difference between a mirror and a white surface is that a mirror is perfectly flat and therefore will reflect light at always the same angle. It may absorb some of it, or even most of it (black mirror!) but what it reflects, it will do it always the same way. A white painted surface can be thought as a surface covered in tiny mirrors, all pointed in a different direction, and depending on which tiny mirror a ray of light will hit, it will be reflected in a different direction. With large pieces, like with a broken mirror, you will still see (as you said) scrambled images. But because the pieces are so tiny, you won't be able to make out an image and you will only see white. A glass pane can look like a mirror, but reduce it to a powder and it becomes white. It has nothing to do with absorption, you can have a 1% reflective mirror, and 100% reflective diffuse white. The way the "tiny mirrors" that make up the white paint are oriented give out its general look. Have them oriented mostly with the surface and you get a glossy finish, have them randomly oriented in many directions and you get a matte finish (this is called specular reflection btw). I've seen mirrors be called "smart white", it is white because it reflects most light in most wavelengths, and it is smart because it keeps the rays of light oriented.