Shorter-wavelength light is absorbed by molecules with smaller gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
Lycopene (C40H56) has a highly conjugated double bond system: 11 double bonds in conjugation. This is a repeating sequence of 11 double bonds, each separated by a single bond. This large conjugated structure has a very small gap between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The energy needed to excite an electron from the HOMO to the LUMO in lycopene is so small that the wavelength of light needed to excite the electron appears in the visible spectrum. In exciting an electron from the HOMO to the LUMO, lycopene absorbs blue-green light photons and therefore appears reddish-orange. Lycopene is the red pigment color found in tomatoes and in many other reddish-orange vegetables.
Double-bonded carbon atoms are bound together with π and σ bonds. The loosely held electrons in the double bond (π bond) result from an overlap of p orbitals on the adjacent carbon atoms. Orbitals are representations of wave functions. Two positive amplitudes combining result in constructive interference. Positive and negative amplitudes combining result in destructive interference. Constructive interference creates an energetically favorable π bonding molecular orbital, while destructive interference creates an energetically unfavorable π antibonding molecular orbital.
Larger molecules with multiple double bonds result in more smaller HOMO and LUMO gaps and the absorption of photons from longer wavelengths. Lower-energy light is used to excite electrons in conjugated systems. The energy of light increases across the visible spectrum (red, orange, yellow, green, blue, indigo, and violet), with red being the lowest energy with the longest wavelength and violet the highest energy with the shortest wavelength. Other chemicals that absorb in the visible spectrum include transition metals and organic dyes.