Final answer:
Robert Hooke invented the first optical microscopes and viewed single-celled organisms.
Step-by-step explanation:
Robert Hooke invented the very first optical microscopes and was the first person to view single-celled organisms. He coined the term 'cell' to describe the basic building blocks of life. Hooke's observations and drawings of cells in his book 'Micrographia' laid the foundation for modern cell theory.
Learn more about invention of optical microscopes and discovery of single-celled organisms