Light : as we all know is one of the most important and valuable resources on earth. Imagine a world without light, no! You don't want to be in such a world. This could mean no sign at all equivalent to being blind. 
Let's imagine you are looking for an object in the dark,  you definitely will need a source of light,  which without it would make your aim almost impossible. While looking for your object,  someone points a torch directly to your eyes, the outcome you know already. 
You'll try to look away from the torch light irrespective of the fact that thou needed light to find you object (uncomfortable) 
This is to summarize that the right things used in the wrong way could be uncomfortable and useless. ![Dark-and-Light.jpg](https://cdn.steemitimages.com/DQmPayLLHsUFMHQ2nJzXU2hbutMDiMhwZq6KdpPf6YFKPfH/Dark-and-Light.jpg)