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11-cis-retinal is a geometrical form of retinal with a cis configuration at the carbon-11/carbon-12 double bond. In vertebrate rod rhodopsin, it acts as the bound chromophore that absorbs light. Photoisomerization toward all-trans-retinal triggers a change in the receptor's state, linking molecular geometry to photoreceptor signaling.^[ref]

The name identifies a configuration, not a different number of atoms. 11-cis-retinal and all-trans-retinal have the same molecular composition but differ in the arrangement around a double bond. Replacing a cis arrangement by trans is therefore an example of cis-trans isomerism, rather than cleavage of the retinal molecule into smaller fragments.

The $2.8$ angstrom rhodopsin crystal structure showed the retinal-binding pocket in the dark-state receptor. Bound 11-cis-retinal helps maintain its inactive arrangement; its behavior depends on the surrounding protein, so free retinal and retinal inside a receptor need not have identical optical properties.^[ref]

The visual cycle regenerates the usable 11-cis supply. In RPE65-deficient mice, 11-cis retinoids and normal rod pigment were absent despite retained rod opsin. This distinguishes making the receptor chain from supplying the correctly configured light-sensitive component.^[ref]

A light-sensitive receptor protein associated with a chromophore. Sequence, expression, chromophore and neural processing all contribute to visual performance.

Bleaching powder is made by passing chlorine over slaked lime, exposing a large surface of the powder to the gas. The contemporary name “chlorinated lime” is more informative than treating it as pure calcium chloride. The 1867 British Pharmacopoeia (1867) describes this manufacture and a dull-white, partly water-soluble powder with bleaching and disinfecting properties.^[ref]

An idealized overall chemical equation isThe chlorine undergoes disproportionation: one part becomes chloride, with oxidation state $-1$, and another becomes hypochlorite, with oxidation state $+1$. Commercial bleaching powder contains residual calcium hydroxide, water and chlorine-bearing solid phases; the chemical equation is a useful composition balance, not a claim that the powder is one pure molecular substance or simply a fully characterized mechanical mixture. The old formula ${\mathrm{CaOCl}}_2$ is an abbreviated representation, not the formula of pure calcium hypochlorite, ${\mathrm{Ca}(\mathrm{OCl})}_2$.

Its chief properties follow from the hypochlorite chemistry. The aqueous extract is alkaline and oxidizing. The chemical equilibriumproduces hypochlorous acid on lowering the pH. Oxidation of coloured organic compounds can destroy the chromophores responsible for their colour. This is irreversible chemical alteration of a dye, rather than merely washing it away. The historical description that bleaching works through “nascent oxygen” is not a necessary modern mechanism: hypochlorous acid and related reactive chlorine species themselves react with substrates.

With sufficient hydrochloric acid, the hypochlorite and chloride can generate chlorine:This explains acid-induced gas evolution and why acidification is not an innocuous way to use a bleaching material. Exposure, moisture and heating can reduce the stored oxidizing strength. Its useful strength is measured as available chlorine, rather than inferred from the powder's total mass or its smell; inert material and residual lime do not provide the same bleaching capacity.

Photoisomerization is a light-induced conversion between molecular isomers: absorption of a photon initiates a change in molecular arrangement without requiring a change in elemental composition. The resulting isomers can have different shapes, spectra or interactions even though they contain the same kinds and numbers of atoms.

A biological example occurs in rhodopsin. Its bound 11-cis-retinal chromophore changes toward all-trans-retinal after light absorption, initiating changes in the receptor that can activate transducin. This converts a molecular rearrangement into a signaling event.^[ref]

The change is an example of cis-trans isomerism, not fragmentation of retinal into smaller molecules. “Light-induced” also does not mean that every photon entering a sample produces one permanent isomerization. Absorption, competing excited-state processes and subsequent reactions can affect the measured outcome.

An idealized photochemical quantum yield for a specified primary product is $\Phi =N_{\mathrm{product}}/N_{\mathrm{absorbed}\mathrm{photons}}$. If 100 photons are absorbed and 30 product-forming events are counted under a defined low-conversion assay, $\Phi =0.30$. Counting incident rather than absorbed photons would answer a different question. The example is bookkeeping, not a measured rhodopsin yield.

The surrounding protein can influence the chromophore's behavior by controlling its local environment. A free molecule's light response therefore need not be identical to its behavior inside a receptor. The useful mechanism is the full chain from absorption to rearrangement to altered interaction with other molecules.

A light-absorbing aldehyde related to vitamin A, used as a chromophore in rhodopsin-family proteins. Its local packing and bonding connect light absorption to structural changes.