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•• A good microscope can readily resolve objects that are 1 μm in diameter (1 μm is about twice the wavelength of visible light). The unaided human eye can resolve objects no smaller than roughly 0.1 mm = 100 μm. Suppose a 1-μm diameter Brownian particle is observed (with a microscope) to diffuse an average distance of 5 times its diameter (5 μm) in 20 s. Under the same conditions of temperature and air viscosity, another Brownian particle of 100 μm diameter is observed with the unaided eye. How long will this particle take to diffuse an average distance of 5 times its diameter? Why was Brownian motion not discovered until after the invention of the microscope?
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