OK, so yer walking by a bunch of cars on yer way to the local pick-up bar and checkin’ out yer six-pack in the reflection from the glossy paint jobs. And you notice that the white Surburban doesn’t show you squat while the black beamer is ideal for baskin’ in one’s own glory.
Why is this? I remember from school that white surfaces are white because they reflect most of the light rays and black surfaces absorb most light rays (and so are much hotter out in the sun as well). So it seems to me that a white shiny surface should make the best mirror and the black surface the poorest.
White surfaces not only reflect back all visible wavelengths, but they tend to reflect back light in random directions. What’s going on here is twofold. First of all, the glossy surface of the paint (perhaps the clearcoat) is reflecting back light kind of like a mirror (i.e. not all random-like). The property of the paint that makes it appear white, however, is reflecting light back in random directions. The result is that the randomness of the white paint washes out the reflection.
So it’s not that black paint has more mirror-like properties. Just that it doesn’t wash out the light that it does mirror back.
Hmmmm, that’s kinda what I was thinking it might be: that the black surface didn’t reflect a lot of extraneous stuff. So the difference between a white surface and a mirrored surface is that the white surface reflects lights back in random directions and a silvery surface reflects back accurately?
So, I guess a related question is why only metallic surfaces have a true mirror reflection. Is it an inherent property of the metal (I’m imagining something to do with electrons but it’s a really wild guess) or is it related to smoothness of the material?
A little from column A, a little from column B. A “rough” surface, meaning a surface with imperfections on the scale of the wavelength of light (about 1 micrometer to within an order of magnitude) will scatter the light rays every which way, and you won’t get a surface you can see your reflection in.
The reason metals are such good reflectors, though, is that some of their electrons aren’t bound to any particular atoms — they can move around freely within the metal. As you may be aware, light waves are really just a special kind of electromagnetic field; also, these waves are created by the movement (specifically, the acceleration) of electric charges. So when the electromagnetic field hits the free electrons in the metal, the electrons move around in such a way that the original wave is cancelled out and a new wave (exactly replicating the first) is created heading away from the metal.
This is all very hand-wavy, but I hope it helps.
Not really true – semiconductors make pretty good mirrors, and you can make dielectric stacks with no metal in them that are perfect reflectors.
But it is true that only metals, of commonly-available things, make really good mirrors. They have a high real part of the refractive index, and the reflection coefficient at normal incidence is (n-1)**2/(n+1)**2. For glass, with n about 1.5, that’s only 4% reflection. Water, with n = 1.33, is even worse. For most metals, the real part of the refractive index is 2 or higher. See Born and Wolf’s Principles of Optics, chapter 13, for the excruciating details. (you have to include the imaginary part in the calculation of reflectivity as well, but it’s the high real part that does the trick).