be invisible to the naked eye

To be imperceptible by normal vision that has not been enhanced or augmented in any way. Because bacteria are invisible to the naked eye, it was only relatively recently in human history that we even became aware of their existence. They make the fine print so tiny that it's nearly invisible the naked eye. The planets are practically invisible to the naked eye. But when viewed through powerful enough telescopes, you can actually make out a remarkable amount of detail.