Molecules containing conjugated system of pi electrons absorb light closer to the visible region than saturated molecules with isolated double or triple bonds.Longer the conjugated system the longer the wavelength of the light absorbed.
For instance ethylene (H2C=CH2) absorbs light of wavelength in the far ultraviolet region of the emr at 180nm
1,3-butadiene(CH2=CH-CH=CH2 with two conjugated double bonds absorbs at 217 nm and is closer to the visible region
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