Since energy cannot be created nor destroyed, the change in energy of the electron must be equal to the energy of the emitted photon.
The energy of the emitted photon is given by:

where
h is the Planck constant
f is the photon frequency
Substituting

, we find

This is the energy given to the emitted photon; it means this is also equal to the energy lost by the electron in the transition, so the variation of energy of the electron will have a negative sign (because the electron is losing energy by decaying from an excited state, with higher energy, to the ground state, with lower energy)
The answer is true. All the galaxies in the universe follow
the law of gravity.
<span>Based from the book, It's about Time: the Illusion of
Einstein’s Time Dilation Explained, </span>
Einstein had explained that all the heavenly bodies in the
universe follow the same scientific laws that are similar to our solar system. The
stars and planets are held by the principles of inertia and gravity