Final answer:
The smallest particle visible to the human eye is about 100 micrometers (µm). Most microbes are smaller than this and require magnification to be seen. For example, bacterial cells are typically about 1 µm in diameter, and viruses are even smaller.
Step-by-step explanation:
The smallest particle visible to the human eye is generally considered to be approximately 100 micrometers (µm) in size. Just to provide some perspective, a typical human red blood cell measures about eight micrometers (µm) in diameter; the head of a pin is about two thousandths of a meter (millimeters, or mm) in diameter. Understanding these scales emphasizes the considerable size of a particle which is still just visible to the unaided human eye.
Most microbes, such as bacteria and viruses, are many times smaller than this and hence require some form of magnification to be seen. For instance, a bacterial cell is typically about 1 µm and viruses can be ten times smaller than that. Hence, special instruments like microscopes are essential to view these tiny organisms and structures.
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