Reflectivity
Also: Reflectance, Degree of Reflection, Reflexionsgrad
The reflectivity (reflectance) of a metal indicates what proportion of the incident light its surface reflects back, and is responsible for the characteristic metallic lustre of gold, silver, and other metals.
Reflectivity (also: degree of reflection or reflectance) describes the proportion of incident electromagnetic light that a material surface reflects back without absorbing or transmitting it. It is expressed as a dimensionless value between 0 and 1 — or as a percentage — and is strongly wavelength-dependent.
Physical Background
When light strikes a polished metal surface, photons interact with the free electron gas of the metal. 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 (e.g. silver and platinum). Gold, by contrast, absorbs short-wavelength blue components more strongly — hence its characteristic yellow colouration.
The fundamental formula for normal incidence is (Fresnel):
R = |( n - 1 ) / ( n + 1 )|²
where n is the complex refractive index of the metal (n = n₀ + i·k, with extinction coefficient k).
Comparison of the Precious Metals
| Metal | Reflectance at 550 nm | Notable feature |
|---|---|---|
| Silver | ≈ 99% | Highest value of all metals in the visible range |
| Rhodium | ≈ 80% | Hence used as rhodium plating on jewellery |
| Platinum | ≈ 73% | Uniform across the spectrum |
| Gold | ≈ 70–73% | Falls off in the blue/green range → yellow tone |
| Palladium | ≈ 67% | Matte grey lustre |
| Copper | ≈ 50–55% | Red-orange shimmer (in the red > 90%) |
Practical Significance for Precious Metal Buyers
Authenticity testing: A freshly polished silver piece reflects back almost all visible light. Abnormal dullness or colour deviations can indicate counterfeits or inferior alloys — as a supplement to density measurement and X-ray fluorescence analysis.
Industrial application: Silver is used in solar mirrors, telescopes, and high-end coatings because of its outstanding reflectivity. Gold infrared mirrors are used in aerospace and medical technology, since gold reflects long-wavelength IR radiation (> 700 nm) at over 97%.
Optical coin quality: For proof coins (proof finish), the surface is polished to a high gloss for maximum reflectivity — an essential quality feature alongside fineness and gross weight.
In Brief
Silver is the strongest reflecting metal in the visible range, at up to 99%, followed by rhodium, platinum, and gold — a physical characteristic with practical relevance from authenticity testing through to solar energy.