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Name: Louie
Status: other
Grade: other
Location: NJ
Country: N/A
Date: October 2006

Question:
What is the most reflective material or surface in the world? Is there a scale of reflectivity, like the Mohs scale of hardness?



Replies:
Louie,

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!

Matt Voss


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.

Vince Calder



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