Reflectivity Materials and Scale
Date: October 2006
What is the most reflective material or surface in
the world? Is there a scale of reflectivity, like the Mohs scale
It is hard to answer this question because "most reflective" can
mean more than one thing. Reflectance in general means that some
wavelength of electromagentic radiation (EMR, usually visible light)
is going from one medium (usually air) into another medium (glass,
water, diamond, etc.) and then bounce back out and returned at a
specific angle. This angle is set by the wavelength of light and
the intrinsic properties of the two mediums which the light travels though.
The issue with trying to define the "most reflective" surface, is
that you have to specifically define the two mediums and the wave
length of light. Even visible light has a wave length span of
400-800 nanometers or so. Each wavelength responds differently when
passing through into a different medium. Quartz will actually
separate out all of the individual wavelengths and create a
rainbow. You know this as the prism effect. Your skin might
reflect a bit of visible light, but X-rays go right through your
skin, where bone has a bit higher reflectance rate for that
wavelength--hence why X-rays can help doctors. Then you can take
some of the huge satellite dishes that can reflect radio waves. The
wavelength of radio waves can be so long that you do not even have
to have a solid surface to reflect the wave. Most very large
satellite dishes are made of a mesh of metal instead of a smooth solid surface.
If you draw a straight line on a sheet of paper that represents the
transition between two media, then you can draw the approach of an
incoming light wave. Once it hits the medium (the first line you
drew) it will change angles. There will be a defined angle between
the ray of light in the first medium and the ray in the second
medium. These are called the angle of incidence/angle of
reflectance and math can be used to solve them.
Think of a diamond. The light enters, usually from the top surface,
and bounces around the bottom until it comes out the sides or top
again. There are also materials like optical fiber (fiber optics)
which has very near 100% internal reflectance. This reflectance is
so high, that the integrity of data can be kept intact over long
distances. This is because once the light enters the medium, when
it approaches the end of the medium again to exit the medium, it is
reflected back internally. This is an extreme example of
reflectance angle. These latter examples might be hard to
comprehend without some simple pictures being drawn.
To make things further complicated, I have only described surfaces
that have high transmission rates. Other opaque surfaces like
brick, stone, walls, metals etc do not transmit most EMR all the way
through the medium. In these cases there is surface reflectance and
absorption. Absorption at specific wavelength and reflectance at
others is what gives color to the object that we look at. A red
object reflects red light and absorbs most/all other visible
light. Here again, though, you need to specify what wavelength of
light you are dealing with to come up with a "most reflective"
surface. Mirrors are very good at reflecting most visible light
without separating out the color. I would say that highly polished
mirrors are the best general reflective objects that we have.
There is a scale for reflectivity and it can be measure in either
percent or decibels. The percent reflected over the percent
absorbed results in the reflectivity of an object. Mirrors are
generally over 99% reflective. But remember, fiber optic cable has
an internal reflectivity of 99.99+ % depending on the quality of the cable.
I hope this helped clarify things instead of make things more confusing!
A reflectivity scale is much trickier to define than a hardness
scale -- not that hardness is all that easy because, for example a
material may be hard in one direction and soft in another. In the
case of reflectivity, that changes with the angle of incidence, the
interference of thin layers on the surface, the wavelength of the
radiation, and the list goes on. In particular the reflectivity is
usually optimized for a desired wavelength range which compromises
its reflective properties in another range. The cleanliness of the
surface is also a major factor. There are just too many variables to
just give "the world's greatest" answer.
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Update: June 2012