Read the link below and feel free to follow up any of the links from those pages.
Do a summary of the page in green in your MasteringPhysics account, under assignment R130508.
If you found something unclear or confusing, ask a question that might help clarify for you. If there was nothing unclear or confusing, pose a question that will help you relate the information in the page to other things you know or will clarify for you how it might be relevant or important to other topics.