Electrons cannot simply exist anywhere they want around an atomic nucleus. They must sit in specific orbitals that have exact energy values. An electron can only jump if a photon exactly matches the energy gap between two orbitals.
Chemists calculate this required energy using the formula ΔE = hν = hc/λ. This strict rule explains why glowing gases produce sharp lines of color instead of a continuous rainbow. Molecules experience several common types of electron jumps when they absorb light.
These specific types include the σ→σ*, n→σ*, π→π*, and n→π* transitions. Each specific jump requires a different amount of photon energy to succeed. Quantum physics uses strict selection rules to predict if a jump will occur.
These selection rules look at the electron spin and the overall orbital symmetry. Jumps that follow all the selection rules are called allowed transitions. Allowed transitions happen almost instantly and absorb or release light very strongly.
Jumps that break the selection rules are given the label of forbidden transitions. Forbidden transitions are highly unlikely, so they happen slowly and show weak light interactions. This slow release of trapped light energy causes the lingering glow seen in phosphorescent materials.