Selection of keywords

Keywords are those phrases which are responsible for the searching of the post. Let's suppose there is a post on "Light" and we want user to see our past when he/she searches about light. Now write the keywords by keeping in mind that what phrases user can use to search about light.

For example "what is light" now this is one keywords you have to imagine what phrases user can search.

Keywords for light : light,what is light,what is speed of light,photon,what are photons