A nanometre is one-billionth of a metre, and 400 nanometres is a specific measurement often associated with light. This wavelength falls within the visible spectrum, appearing as a shade of blue-violet light. In the context of photography and optics, 400 nanometres is significant because it represents the edge of the visible light range. Light at this wavelength can influence various processes, including photosynthesis in plants and the behavior of certain materials under ultraviolet light.