Definition:

with the naked eye

With only the eyes and without the help of instruments such as   telescopes or microscopes

e.g., Enough light gets through space for us to see thousands of stars with the naked eye and millions with an optical telescope.

Some 2,500 stars can be seen with the naked eye.

Residents could see enemy warships with the naked eye.

Wall Street Journal (2023)

Times, Sunday Times (2013)

Translation to Brazilian Portuguese:

A olho nu.