James Clerk Maxwell, in 1861–64, published his theory of electromagnetic fields and radiation, which shows that light has momentum and thus can exert pressure on objects.
SI units :
ENERGY/SPACE = [ Joul / cubic meters = Newton *meter / cubic meters = N/m^2 ]
1kg Ball VS 1kg ( 100 m^2 metal sheet )
1kg ( wood ) and 1kg ( steel )
wood is more light so ... has got biger surface !!!