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How can absorbed light move an electron from a bonding orbital to an antibonding orbital?

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Diagram showing the formation of a polar covalent bond in a water molecule, with two hydrogen atoms bonding to one oxygen atom to form H2O, illustrating shared electrons and chemical reaction. — covalent bond In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more time in the vicinity of the other atom. The effect of this orbital distortion is to induce regional net charges that hold the atoms together, such as in water molecules.

According to molecular-orbital theory, electrons in a molecule occupy specific energy levels known as molecular orbitals. A covalent bond typically forms when electrons occupy a lower-energy bonding molecular orbital, which represents a region of space enveloping the bonded atoms.1

An electron can move from this lower-energy bonding orbital to a higher-energy antibonding orbital through a process called electronic excitation. This transition occurs when the molecule absorbs energy, such as from visible or ultraviolet light. The energy required for this promotion is defined by the equation $Δ E = h ν$ , where the energy difference between the orbitals corresponds to the frequency of the absorbed light.12

When a molecule absorbs light of the appropriate wavelength, an electron is promoted to an excited state in the higher-energy, unoccupied antibonding orbital. This absorption of specific light frequencies is what determines the observed color of many substances, as the molecule reflects the remaining light that was not absorbed.12