What physical properties are manifested when an electron leaves Na and moves to Cl to form an ionic bond?
The light and heat are created by the exothermic nature of the reaction.  If you look up "Born Haber cycle sodium chloride" you will see information on why the reaction is exothermic.

The reaction "needs help" because of the fact that the sodium will likely be coated with a layer of sodium oxide or hydroxide that will not allow chlorine to collide with the sodium metal.  Even then, with pure sodium metal the reaction will be faster if heated as any reaction needs activation energy to proceed.  Adding water causes an exothermic reaction that can provide activation energy for the reaction with chlorine.