"Optical wavelengths" refers to the range of light that the human eye can perceive. It includes the colors we see such as red, orange, yellow, green, blue, indigo, and violet. Full definition
The results showed that the planet is extremely dark at optical wavelengths. (sciencedaily.com)
This helps clarify earlier work, which shows that UV and optical wavelengths appeared to be located far from where the black hole's tidal influence would destroy the star. (scientificamerican.com)