Titanium dioxide is known to be a white pigment that reflects a significant amount of light across the visible spectrum. However, when titanium dioxide appears black, it means that it is absorbing a substantial portion of light rather than reflecting it.
The specific wavelength of light that is absorbed by titanium dioxide can vary depending on various factors such as the crystalline structure, particle size, and presence of impurities. Generally, titanium dioxide absorbs light in the ultraviolet (UV) range, particularly in the shorter wavelengths.
In the visible spectrum, titanium dioxide is not known to selectively absorb any particular wavelength to the extent that it appears black. Instead, the black appearance of titanium dioxide is typically due to the formation of a dense and opaque layer, which absorbs or scatters light across the entire visible spectrum, resulting in a lack of reflection.
It's worth noting that the behavior of titanium dioxide can be modified through various means such as doping or structural modifications, which may lead to selective absorption in certain wavelength ranges. However, the specific conditions required to achieve such effects would depend on the intended application and the desired optical properties.