Apparent magnitude of a star is a measure of its brightness in logarithmic units. By conventional definition, apparent magnitude mm is related to the observed flux density F of light from the star (which is its brightness) by the formula
where is the reference flux density corresponding to zero apparent magnitude. Since the flux density (brightness) for a star can be measured in different spectral bands (ultraviolet, visible, infrared etc.), one can speak of the apparent magnitude in the corresponding spectral band. For two objects A and B with apparent magnitudes and , respectively, we have
Thus, for and , we obtain , or . Hence, the ratio of their brightness is .
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