Reflectivity
Also: Reflectance, Reflection factor, Reflecting power
The reflectivity 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: reflectance or reflection factor) 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 colour.
The basic 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 precious metals
| Metal | Reflection factor at 550 nm | Special feature |
|---|---|---|
| Silver | ≈ 99% | Highest value of all metals in the visible range |
| Rhodium | ≈ 80% | Hence used for rhodium plating on jewellery |
| Platinum | ≈ 73% | Uniform across the spectrum |
| Gold | ≈ 70–73% | Drop in the blue/green range → yellow tone |
| Palladium | ≈ 67% | Matt-grey lustre |
| Copper | ≈ 50–55% | Red-orange shimmer (in the red > 90%) |
Practical significance for precious-metal buyers
Authenticity testing: A freshly polished piece of silver reflects back almost all visible light. Abnormal dullness or colour deviations can indicate counterfeits or inferior alloys – complementary to density measurement and X-ray fluorescence analysis.
Industrial application: Silver is used for its outstanding reflectivity in solar mirrors, telescopes and high-end coatings. Gold infrared mirrors are used in aerospace and medical technology, since gold reflects long-wave IR radiation (> 700 nm) at over 97%.
Optical coin quality: In proof coins the surface is high-gloss polished 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 property that has practical relevance from authenticity testing to solar energy.