The free-electron model represents these MOs as particle-in-a-box wavefunctions , with orbital energies , (). The length of the box is taken as , where is the average bond length, typically on the order of 1.40 Å. One half a bond length is added to each end of the molecule to simplify this formula. Taking into account the Pauli principle, two electrons (with and spins) are fed into each of the available orbitals of lowest energy. The highest-occupied molecular orbital (HOMO) then has . In the lowest-energy electronic transition, one HOMO electron is excited to the lowest-unoccupied molecular orbital (LUMO), with , producing a band with its maximum wavelength in the ultraviolet or visible region. Using , we find (a useful constant is the Compton wavelength ). The prediction for butadiene is 207 nm, which agrees closely with the observed absorption band at in the ultraviolet. The increase of with is correctly predicted; however, quantitative results for the higher polyenes are not very accurate. As the absorption band begins to impinge on the visible region of 400–700 nm a polyene becomes colored. A chain of 10 conjugated carbon atoms, as contained in retinol (vitamin A), shows a pale yellow color.

It is sufficient to compute the modified energies using first-order perturbation theory, such that . The wavelength formula thereby generalized to . With appropriate choices of the empirical parameters, much better agreement can be obtained for the absorption frequencies over a large range of polyenes, as shown in the second snapshot.