For butadiene-1,3 energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is smaller than in ethene. Addition of successive conjugated double bonds progressively narrows the gap between HOMO and LUMO.Thus longer wavelength (lower energy) radiation required for the transition
3D picture of molecular orbital diagram is available on link: http://www.chemtube3d.com/orbitalsbutadieneUV.htm
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