This world is a mixture of good and evil, light and darkness. The whole world is shade, there is no pure light in the world. The only reason you can see right now is a mixture of light and the absence of light. If it was pure light you would be blind. If it was pure darkness you would be blind. So the whole world is a mixture of light and darkness. Now what Allah is telling is that He created this world to take you from darknesses into light so the process of being in the world is a process of moving through shades. It is a process of coming from darkness into the light but that is a process. He is taking you out of darkness so there are different degrees of darknesses to the one light which is the light of guidance. This is an evolution so human beings if they are on a spiritual are evolving, things are becoming cleaer because more light is coming in. More light is being introduced into the darkness of the material world. So this is the penetration of the spiritual realm into the material realm. This is what revelation in essence is. It is the movement from the unseen into the seen from that pure light, it is light moving into the darkness of the material world. Because the material world in a sense not pure otherness but because it is other than God because it is corruptible because it has elements that are suscepitble to corruption. The degree to which these elements exist in the world is the degree of darknesses in the world because it is other than God.