In atoms, electrons can't exist between energy levels—they occupy discrete allowed states like rungs on a quantum ladder. When a photon's energy exactly matches the gap between two levels, electrons jump. Drag the slider to change which transition occurs and watch the photon energy adjust.
When a photon with exactly the right energy strikes an atom, an electron absorbs it and jumps to a higher energy level instantaneously. The photon energy must precisely match the gap—even slightly wrong and nothing happens. This is how cool gas clouds absorb specific wavelengths from starlight, creating dark absorption lines in stellar spectra. Each element removes its unique set of wavelengths, acting as a cosmic barcode. The Lyman series corresponds to transitions starting from the ground state (n=1), producing ultraviolet photons invisible to our eyes but detectable by space telescopes.
Electrons cannot exist between energy levels, only at discrete allowed values—when a photon's energy exactly matches the gap, absorption moves electrons up and emission releases a photon as they drop, with frequency determined by hf = ΔE.