If you are reading an article 4 or 5 times without remembering it, you are not reading it correctly. Reading for retension is an invaluable skill and pretty much a prerequisite for learning anything in a text-based environment. I have found that this tends to stem from one broad issue - not caring about what you are reading. Did you read it too quickly? Skip words and phrases? Listen to music, talk to your friends? It's like a conversation; if you do not want to recollect it you will not be able to. Next time you read through it, be sure to treat it as though it's important (and the basics of light, as simplified as this guy writes about them, are pretty important).