The words phosphorescence, phosphorescents sound the same but have different meanings and spellings. Why do phosphorescence, phosphorescents sound the same even though they are completely different words?
The answer is simple: phosphorescence, phosphorescents are homophones of the English language.
Persistent emission of light following exposure to and removal of incident radiation.
Emission of light without burning or by very slow burning without appreciable heat, as from the slow oxidation of phosphorous: "He saw the phosphorescence of the Gulf weed in the water” ( Ernest Hemingway).
Plural form of phosphorescent.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.