Reflectivity
Also: Reflectance, Degree of reflection, Reflection factor
The reflectivity of a metal describes what proportion of incident light its surface throws back, and it is responsible for the characteristic metallic lustre of gold, silver and other precious metals.
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Reflectivity (also called reflectance or the degree of reflection) is the fraction of incoming electromagnetic light that a material surface throws back rather than absorbing or transmitting. It is expressed either as a dimensionless value between 0 and 1 or as a percentage, and it depends strongly on wavelength.
The physics behind it
When light strikes a polished metal surface, photons interact with the metal's free electron gas. The collective oscillation of these conduction electrons (plasma oscillation) determines which wavelengths are absorbed and which are reflected. If the plasma frequency lies above the visible spectrum, the metal appears silvery-white, as with silver and platinum. Gold, by contrast, absorbs short-wavelength blue components more strongly, which gives it its characteristic yellow tone.
The basic Fresnel formula for normal incidence is:
R = |( n - 1 ) / ( n + 1 )|^2
where n is the complex refractive index of the metal (n = n0 + i*k, with extinction coefficient k).
Comparing the precious metals
| Metal | Reflectivity at 550 nm | Notable feature |
|---|---|---|
| Silver | approx. 99% | Highest value of any metal in the visible range |
| Rhodium | approx. 80% | Used as rhodium plating on jewellery |
| Platinum | approx. 73% | Even across the spectrum |
| Gold | approx. 70-73% | Falls off in the blue/green range, giving a yellow tone |
| Palladium | approx. 67% | Matt-grey lustre |
| Copper | approx. 50-55% | Red-orange sheen (over 90% in the red) |
Practical relevance for precious metal buyers
Authenticity checks: a freshly polished piece of silver throws back nearly all visible light. Abnormal dullness or colour deviations can point to forgeries or inferior alloys, complementing methods such as the density test and X-ray fluorescence analysis.
Industrial use: thanks to its outstanding reflectivity, silver is used in solar mirrors, telescopes and high-end coatings. Gold infrared mirrors are used in spaceflight and medical technology because gold reflects long-wavelength IR radiation (over 700 nm) at more than 97%.
Optical coin quality: for proof coins the surface is polished to maximum reflectivity, an essential quality feature alongside fineness and gross weight.
In a nutshell
Silver is the strongest reflecting metal in the visible range, reaching up to 99%, followed by rhodium, platinum and gold. It is a physical property with practical relevance ranging from authenticity testing to solar energy.