Bismuth’s range of colours and iridescent shine is one of the things that makes bismuth such a valuable addition to the collections of artists and geologists alike. The cause of these wonderful colours is an interaction of the light waves bouncing off of the bismuth’s surface layer. This interaction, called thin film interference, is also responsible for the waves of colour you see in oil floating on water.
Thin film interference is the result of the interactions of light waves within layers of different materials. For example, when you add oil to water, the oil makes a thin film on top of the water. When light hits the two layers of material, it goes slower through the oil than it does through the water, which results in the light reflecting off of each surface at a different angle. These different angles of the returning waves of light mean they can then intersect and interfere with each others’ wavelengths. Sometimes the wavelengths are intensified and other times they cancel each other out and are dimmed. This interference pattern changes depending on the material the light is passing through and the thickness of the film, both of which can affect the angle of the light passing through. Since a thin film is rarely a uniform thickness, slight variations in the thickness of the film can cause different wavelengths to emerge. The wavelength of light is what determines what colour we perceive it as, and this is why we see different bands of colour when we look at the oil.
The same phenomenon can be seen in bismuth. When bismuth is cooled while exposed to the air, the surface layer of bismuth reacts with oxygen to form a thin film of bismuth oxide. The slower the bismuth is cooled, the thicker the bismuth oxide layer will be. The bismuth oxide layer itself is clear, but the thickness of this film determines what colour the bismuth appears. The thinner bismuth oxide layers produce colours with shorter wavelengths such as blue and purple. The thicker bismuth oxide layers produce longer wavelengths like yellows and oranges, but producing much longer wavelengths such as red or infrared can become complicated as the film becomes too thick. If you can control the thickness of this oxide layer carefully, you can choose what colour the bismuth appears, and a gradual change in the wavelength results in a beautiful rainbow gradient.
Some examples of The Bismuth Smith products, their coloring and corresponding wavelengths
Thin films have a range of useful applications, especially in the fields of optics and optoelectronics. Thin film coatings are applied to the lenses of eyeglasses, binoculars and telescopes, to reduce glare and reflections, and can be used in optoelectronic devices that detect and control light. Bismuth oxide thin films in particular are studied often because they have unique properties such as a high refractive index, semiconductivity, and a large energy bandgap. Bismuth oxide thin films can also be used in humidity and gas detecting sensors, and ceramic glass coatings.