Gold leaf consists of pure gold hammered into extremely thin sheets, a process known as goldbeating. This ancient technique produces a material typically around 100 to 125 nanometers thick. For centuries, gold leaf has served as a decorative medium, frequently used in art, architecture, and the embellishment of objects through gilding. Its traditional applications range from illuminating manuscripts to adorning statues and architectural elements, where its lustrous appearance is highly valued.
Reflected Appearance
When light strikes the surface of gold leaf, it exhibits the familiar bright, metallic golden color characteristic of bulk gold. This is the appearance observed in most gilded objects. The golden hue arises because gold inherently reflects a high percentage of yellow, orange, and red wavelengths of visible light. The interaction of light with the delocalized electrons within the metal’s structure causes this strong and preferential reflection, giving gold its distinct warm color.
Transmitted Appearance
Holding gold leaf up to a light source reveals an optical phenomenon. Instead of its usual golden reflection, light passing through the ultra-thin gold leaf appears as a greenish-blue or bluish-green. This color shift is a direct consequence of the gold leaf’s extreme thinness, which allows some light to transmit through the material rather than being entirely reflected. The transmitted color contrasts with the reflected golden color. This effect is observable when the gold film is approximately 50 nanometers thick or less.
Why Gold Leaf Changes Color
The color change of gold leaf from reflective gold to transmitted greenish-blue is due to interactions between light and matter at the nanoscale. Gold leaf’s extreme thinness causes light to behave differently than with thicker gold. At this minute scale, gold selectively absorbs certain wavelengths of light while allowing others to pass through. Specifically, gold preferentially absorbs the red, orange, and yellow portions of the white light spectrum. This selective absorption means that the remaining transmitted light is primarily composed of the green and blue wavelengths, which then become the perceived color.
This phenomenon is closely tied to the electron interactions within the gold, particularly a concept known as surface plasmon resonance. Surface plasmons are collective oscillations of electrons on the metal’s surface that are excited by light at specific wavelengths. In very thin gold films, these plasmonic effects influence which wavelengths are absorbed and scattered, and which are transmitted. The strong absorption of yellow and red light allows green and blue light to pass through, creating the characteristic transmitted hue.
Where This Property Matters
The optical properties of gold leaf have found applications across various fields, historically and in modern technology. In historical art, particularly medieval illuminated manuscripts, gold leaf was frequently used for decorative elements and backgrounds. The subtle greenish-blue translucency, when light passed through the pages, could add depth and a dynamic quality to the artwork, enhancing its visual appeal.
Beyond aesthetics, the specific light interaction with thin gold films has practical uses in scientific instruments. Gold’s ability to strongly reflect infrared light, even when semi-transparent to visible light, makes it suitable for infrared shielding, such as in visors for heat-resistant suits or spacesuits. In modern nanotechnology and optics, the plasmonic properties of thin gold films and gold nanoparticles are being explored for advanced applications. This includes the development of highly sensitive biosensors, efficient catalysts, and novel electronic components, leveraging gold’s precise light absorption and transmission characteristics at the nanoscale.