Gold leaf is a material used for thousands of years as a decorative covering in art and architecture. It is created by hammering pure gold into sheets approximately 100 nanometers (nm) thick. The optical properties of any material change dramatically when its physical dimensions shrink to the nanometer scale. This extreme thinness means gold leaf interacts with light in two fundamentally different ways, depending on whether the light reflects off or passes through the material.
The Color of Reflected Light
When you look at a piece of gold leaf, the color you see is the result of light reflecting off its surface, which appears as the yellow-gold hue. This metallic luster is a result of the free electrons within the gold structure absorbing and re-emitting light energy. The electronic structure of gold causes it to absorb light in the blue and green regions of the visible spectrum more strongly than most other metals. Since blue and green light are absorbed, the reflected light is dominated by the longer wavelengths, which include the yellow, orange, and red portions of the spectrum.
The combination of these longer wavelengths results in the characteristic golden color we recognize on gilded objects. The reflective properties of gold leaf are largely identical to those of bulk gold because the interaction occurs at the surface layer.
The Surprising Color of Transmitted Light
The optical paradox of gold leaf emerges when the material is held up to a bright light source, allowing light to pass directly through the sheet. The color transmitted through the ultra-thin metal is a distinct bluish-green or greenish-blue, which is unexpected for a material that appears yellow when reflected. This phenomenon depends on the gold being beaten down to a thickness of 100 nanometers or less.
The greenish-blue color is the complementary color to the yellow-red light that is strongly reflected and absorbed by the gold. The gold leaf acts like a selective filter, removing a specific portion of the white light spectrum as it travels through the material.
The Physics of Extreme Thinness
The shift in color from reflected yellow to transmitted bluish-green is rooted in the unique way light interacts with matter at the nanoscale. Visible light is composed of electromagnetic waves with wavelengths ranging from approximately 400 nm (violet) to 700 nm (red). When the material’s thickness is comparable to or less than these wavelengths, the traditional rules of bulk material optics no longer apply.
The gold leaf’s thickness of 100 nm is significantly shorter than the wavelengths of most visible light. As white light passes through this ultra-thin layer, the gold atoms’ free electrons interact with the light waves. Gold naturally absorbs and reflects the yellow and red (longer) wavelengths, which explains its reflected color.
Because the path length of the light traveling through the metal is minuscule, only the most efficiently absorbed wavelengths—the yellow and red components—are fully blocked. The blue and green (shorter) wavelengths are the least efficiently absorbed by the gold’s electronic structure, allowing them to pass through the thin layer relatively unimpeded. The transmitted light is composed primarily of these shorter, less-absorbed wavelengths, creating the greenish-blue hue.