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I read that a (sufficiently) white surface reflects more (visible) light than a mirror. Is that true? And if yes, why?

Raffael
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    "A mirror" and "a white surface" are not opposites. You could argue that a mirror is also a white surface - just a white surface that happens to reflect specularly (parallel rays). – Steeven Apr 16 '21 at 10:09
  • Interesting perspective. – Raffael Apr 16 '21 at 10:13
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    To develop the first comment a bit more, you can consider the following thought experiment: Start with a mirror made of a homogeneous material, say stainless steel for instance, then break it into pieces again and again a couple of times, each fragment will reflect light as efficiently as the the initial mirror (albeit in a different direction). If you do this a large enough amount of time, you end up with a white powder, whose reflecting power is the same as the one of the initial mirror (only spread over a large amount of grains). – Serge Hulne Apr 16 '21 at 12:30
  • Hi, please provide the reference which made that statement. It would be helpful to us, as then we could show what assumptions it made about a mirror's reflectivity – Carl Witthoft Apr 16 '21 at 14:06
  • Here’s one possible example: https://patentimages.storage.googleapis.com/c7/70/db/6a3db1bed0860d/US4317855.pdf – Serge Hulne Apr 16 '21 at 15:21
  • More generally, white paint seems to be made of dielectric grains embedded in a filler matrix: https://www.explainthatstuff.com/howpaintworks.html – Serge Hulne Apr 16 '21 at 15:37
  • @CarlWitthoft: let's say you want to grow "tomatos" in a grow box. the intuitive choice for the interior surface is something mirror-like (f.x. tinfoil). but advanced tomato growers recommend a white painted interior. the justification probably varies. reflecting on it a white surface isn't better because it reflects more light but because it disperses it better and reduces shading. – Raffael Apr 16 '21 at 15:41
  • @Raffael understood - but without seeing the article or book in which said farmers make this statement, we can't evaluate what they are using. – Carl Witthoft Apr 16 '21 at 17:36
  • @CarlWitthoft: The answers provided here are very much satisfying thumbs up – Raffael Apr 16 '21 at 18:28
  • It's worth to see another post for readers' further interests. – Ng Chung Tak Oct 03 '23 at 23:36

2 Answers2

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Have you ever wondered why things show colour? Well when light is incident on a surface, light of certain wavelengths are absorbed and we see the complement colours.For example a green surface absorbs light of all colours other than green. But a white surface is a surface that reflects out light of all wavelengths unlike a black surface which absorbs all light.

Now imagine shining a green laser at a mirror . What we get is a reflected green beam.if we repeat this experiment for all other 6 colours we will get similar results.So we can conclude that a mirror reflects light of all colours just like a white surface .

So what makes a mirror different from a white surface?

A mirror possesses specularity i.e parallel rays are reflected parallel to each other thus maintaining an image which does not happen for all white surfaces. A surface which is not a mirror does not reflect the parallel rays in a specular fashion so the reflected rays mix into one homogeneous white light without producing image.

So basically if I take a white surface and give it the property of specularity I get a mirror.

Typically how much light is reflected depends on the surface.For example if you take a sufficiently white surface it will reflect 100 % light but if you compare it with a household mirror that reflects only 80% light then the white surface would reflect more than the mirror.

Möbius
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What we refer to as "white" is actually a large collection of colors. (I'm not talking about "off-white", I'm talking about all the shades we call "white".)

For example, there are hundreds of "white" paints, but they all have different Light Reflectance Values up to 98.1%.

Likewise, there are a lot of different types of surfaces that we call "mirrors".

A household metallic mirror will reflect up to 95% of light. However, dielectric mirrors can reflect up to 99.999% of light.

There isn't really a well-defined lower bound on how much light must be reflected to be considered "white" or "a mirror", but at the upper bound, mirrors are capable of reflecting more light than white paint.

A "sufficiently" white surface could theoretically reflect 100% of light, but I'm not sure anything like that exists in nature or has ever been created.

cowlinator
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