Question

Why is the absorbance of the cell taken near 600nm?

Answer 1

Red wavelengths are transmitted maximally in amber-colored media. Since the bacteria are suspended, and not actually dissolved, in the media, measuring at 600 nm (sometimes 660 nm is used if the spec has a red filter or can read at that wavelength) minimizes the absorbance of the media and corresponds to the reflected and dispersed light interactions of the bacteria.  Alternative wavelengths can also be used if the bacterium displays a specific chromophore.

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