The answer would be it will be longer than the 656.3 nm. The reduced mass of positronium is less than hydrogen so the photon energy will be a reduced amount of for positronium than for hydrogen. So this will mean that the wavelength will be lengthier.
Energy can be one answer! There are many, but energy is a main one.
1)Kenetic Energy is defined as energy which a body possesses by virtue of being in motion. 2)KE) is KE = 0.5 x mv2. Here m stands for mass, the measure of how much matter is in an object, and v stands for the velocity of the object, or the rate at which the object changes its position..
And I hope this helped :)
