In 1912, Giacomo Ciamician published his now famous essay that noted: If photochemistry is adopted by industry 'civilisation will continue as long as the sun shines' -since then, photochemistry has become ubiquitous across both industry and academia. From additive manufacturing to photodynamic therapy, and from organic synthesis to photovoltaics, photochemistry has been central to many technologies that have addressed key societal issues in the 21 st century. These different photochemical processes have one characteristic in common: In order for chemical change to occur, at least one photon must be absorbed by the chemical species -otherwise known as the first law of photochemistry. The propensity of light to be absorbed at a given wavelength -determined by inspection of its absorption spectrum -has long been thought to be the key indicator of how efficiently a photochemical process will progress at said wavelength. Over the past decade this misconception has been initially exposed by our team, and a significant mismatch between the absorbance spectrum of a molecule and its resulting wavelength-resolved photochemical reactivity has been repeatedly demonstrated for a range of photochemical reactions. These findings are based on the photochemical action plot technique jointly developed by the teams of Barner-Kowollik and Gescheidt, a modern-day adaptation of action spectroscopy. Photochemical action plots use monochromatic tuneable lasers to deliver a defined number of photons at each wavelength to a stock solution of the analyte. The wavelength resolved conversion is then determined and subsequently compared to the absorption spectrum of the analyte in question, often revealing a stark mismatch between absorptivity and reactivity. The phenomenon has been observed across many different photochemical reaction classes including photopolymerisations, photo-uncaging, 22 photodimerisations, photoacids and bases, and photochemical depolymerisations. These wide-ranging results from both our team and the literature suggest that a fundamental mechanism is responsible for the mismatch. It is important to note that the profile of the wavelength-dependent reactivity does not simply constitute a bathochromic or hypsochromic shift of the absorption spectrum, but can often uncover a fine structure that is not otherwise visible. When comparing an absorbance spectrum to an action plot, the conditions of the experiment must also be considered. The concentration of a solution and many other factors can all obscure the true wavelength-dependent reactivity; therefore, interrogation of the wavelength-dependent quantum yield is the most reliable way to do so. Wavelengthdependence in the quantum yield indicates a mismatch, and wavelengthindependence indicates that the action plot is not mismatched. In some molecules, little to no deviation between reactivity and absorbance is observed, highlighting that this mismatch is not just a global mechanism and must in part depend upon the system and experimental conditions employed. Additionally, the formation of different photoproducts has been shown to deviate from the absorbance spectrum in unexpected ways. Perspectives on the topic have previously detailed the extent of the mismatch, discussed numerous examples and included a qualitative examination of potential explanations for the mismatch. They have also highlighted the highly beneficial nature of the mismatch for a range of materials science applications. A comprehensive, experimentally-verified theory is yet to be presented in the literature however.