<u><em /></u><em>it is very simple but very important
1 they do not make spelling mistakes...
2 they have a strong grammar concept...
3 they stay away from plagiarism
4 they try to summarize whole topic to make it easy to understand
5 they try to write different topics in comparison... :)</em>
It means the scientific study of sight and the behavior of light, or the properties of transmission and deflection of other forms of radiation