= Hypochromic effect
The \b[hypochromic effect] is a decrease in the intensity of an absorption band, the opposite of the <hyperchromic effect>. It describes absorption strength rather than a shift of the band's <wavelength>.<Hypochromic effect in IUPAC Gold Book>[H02927, Definition]{ref}
Under the <Beer-Lambert law>, $A=\varepsilon c\ell$, so comparing molecular absorption strength requires accounting for <concentration> $c$ and optical path length $\ell$. Diluting a sample decreases its <absorbance> without establishing a decrease in its <molar absorption coefficient>.
An original example keeps concentration, path length and band shape fixed, but reduces the peak <absorbance> from0.80 to0.60. The relative decrease in the peak <molar absorption coefficient> is
$$
\frac{0.80-0.60}{0.80}=0.25,
$$
a25% hypochromic effect. Yet the transmitted fraction increases from $10^{-0.80}\simeq0.158$ to $10^{-0.60}\simeq0.251$. Weaker absorption means more transmitted light, not less.
A useful measurement compares complete spectra in matched <cuvettes> with a <spectrophotometer>, using corresponding solvent blanks and correcting any dilution caused by mixing. Full spectra help distinguish a decrease in band strength from a band moving away from one monitored wavelength. If the corrected peak strength changes while the peak position stays fixed, the measurement directly separates those two effects. A reduced band alone does not uniquely establish the molecular arrangement responsible for it.
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