= Hyperchromic effect
The \b[hyperchromic effect] is an increase in the intensity of an absorption band, for example when a molecule's environment or interactions change. It concerns absorption strength rather than a shift of the band's <wavelength>. Its converse is the <hypochromic effect>.<Hyperchromic effect in IUPAC Gold Book>[Entry H02923, definition]{ref}
At fixed <concentration> and optical path length, the <Beer-Lambert law> gives $A=\varepsilon c\ell$, so an increase in the <molar absorption coefficient> increases <absorbance>. A change of concentration alone can increase absorbance without demonstrating a hyperchromic change in molecular absorption strength.
A motivating measurement is <DNA melting>. In1962, <J. Marmur> and <P. Doty> followed the rise in260nm <absorbance> as DNA was heated. Their apparatus used glass-stoppered quartz <cuvettes> in a Beckman DU <spectrophotometer>, heated by circulating water through thermal spacers. A thermometer measured the cuvette temperature; measurements near the transition allowed about10minutes for equilibration at each step. They corrected absorbance for thermal expansion of the solution and plotted it relative to its value at25degrees Celsius. The midpoint of the hyperchromic rise defined their melting temperature.<Determination of the base composition of deoxyribonucleic acid from its thermal denaturation temperature by J. Marmur and P. Doty>[Printedp.110, section2, apparatus and measurement procedure; Fig.1, experimental transition]{ref}
For an original numerical example, a corrected <absorbance> increase from0.50 to0.65 is a30% hyperchromic rise. Under a stipulated two-state model with linear absorbance mixing and those fixed end-state baselines,
$$
f=\frac{A-0.50}{0.65-0.50}
$$
is the fraction in the high-absorbance state. At $A=0.575$, it gives $f=1/2$. Sloping temperature-dependent baselines or intermediate states require a more detailed model; the30% change is not itself a universal conversion from absorbance to the fraction of melted DNA.
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