Bulb ---> u .............observer B ---> 2u


bulb ---------> 2u ........observer B' ---------> 2u

*observer A

We have long reduction for observer A what about Brightness
more smaller bulb and the same power = more brightness point
more biger bulb and the same power = more darkness bulb


Space around observers = medium for light
inside medium we have some joules nomber

Brightness it is function ( Size ( mm^2 ) and joules no )

Brightness it is additional inforamtion
Observer B' or B is able recognize own motion without outside object observation only measure Joules/mm^2 and Size