Human visual range is generally 400-700 nm, although some small responsiveness remains up to 800 nm and down to 360. Below 400 nm is usually considered ionizing

Human visual range is generally 400-700 nm, although some small responsiveness remains up to 800 nm and down to 360. Below 400 nm is usually considered ionizing radiation. But even a ~20% reflection coefficient can make something appear "opaque" if viewed in sunlight and complete darkness behind it.