Reflectivity
Also: Reflectance, Reflection Coefficient, Optical Reflectance
The reflectivity (reflectance) of a metal indicates what proportion of incident light its surface reflects back, and is responsible for the characteristic metallic lustre of gold, silver, and other precious metals.
Reflectivity (also: reflectance or reflection coefficient) 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-wave blue components more strongly — hence its characteristic yellow colouration.
The fundamental formula for perpendicular 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 Precious Metals
| Metal | Reflectance at 550 nm | Special 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% | Drop-off in the blue/green range → yellow tone |
| Palladium | ≈ 67% | Matt greyish lustre |
| Copper | ≈ 50–55% | Red-orange shimmer (> 90% in the red range) |
Practical Relevance for Precious Metal Buyers
Authenticity testing: A freshly polished silver piece reflects virtually all visible light. Abnormal dullness or colour deviations may indicate forgeries or inferior alloys — supplementary to density testing and X-ray fluorescence analysis.
Industrial application: Silver is used in solar mirrors, telescopes, and high-end coatings due to its outstanding reflectivity. Gold infrared mirrors are used in aerospace and medical technology, as gold reflects long-wave IR radiation (> 700 nm) at over 97%.
Optical coin quality: For proof coins (Polished Plate), the surface is polished to maximum reflectivity — a key quality characteristic alongside fineness and gross weight.
Key Takeaway
Silver is the strongest reflective metal in the visible range with up to 99%, followed by rhodium, platinum, and gold — a physical characteristic that has practical relevance from authenticity testing to solar energy.