Think of it as a sunburn... a really bad sunburn.
But if this relates to your other thread, you really won't be able to buy a 48" fixture that will be too much lighting. If you end up with more watts, or more intense lighting, you'll just have to adjust where you put the corals.
For corals that want less light, if you have a high intensity lighting setup then you'd just put those corals down low in the tank - or under an overhang of rock. For corals that want more light, then those would go up higher. Each coral has its own particular need for light (some more, some less) and you'll need to research those needs before buying or placing them in your tank.