If an electron in an atom gains energy to go from a lower energy level to a higher energy level, it is called Excited energy state.
Ground state energy: The lowest energy state for an electron is called its ground state, which is the energy level it typically resides in. Each electron has a limit on how much energy it can have while still remaining a part of its atom. It is the most stable state of electron.
Excited state energy: An electron is in an excited state when it momentarily possesses an energy state higher than its ground state. If an electron receives additional energy, such as when it absorbs a photon, or packet, of light, or collides with an adjacent atom or particle, it can become excited. As these excited state energy level are unstable. So while coming back to ground state electrons emit radiations suitable wavelength.
Learn more about wavelength of emitted radiation here,
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