why litmus paper changes color ?

Red litmus contains a weak diprotic acid. When it is exposed to a basic compound, the hydrogen ions react with the added base. The conjugated base, formed from the litmus acid, has a blue color, so the wet red litmus paper turns blue in alkaline solution.

Thank u so much .

Once you put acid, the acid will introduce an excess of H+(since it donates a proton, once it is in a water solution), bringing an excess of H+ and will shift the reaction to the left, producing more HA and turning the color red.

A corollary to that question: how does, and to what extent, does bond strength reflect the absorption of light? Bond strength being a general reference to interactions between certain bonds, in this case litmus paper. Or rather a better question is, what can describe the relationship between bond strength and light absorption?

Under acidic conditions, the solution is red, and under basic conditions, the solution is blue. Chemical reactions other than acid-base can also cause a color change to litmus paper