Final answer:
Alkenes and nonconjugated dienes absorb UV light below 200 nm due to their large HOMO-LUMO energy gap, requiring higher energy (shorter wavelength) for electronic transitions.
Step-by-step explanation:
Alkenes and nonconjugated dienes have absorptions below 200 nm because of their electronic structure and the energy required for electronic transitions. In these molecules, the HOMO-LUMO energy gap for a π - π* (pi to pi star) transition is relatively large, meaning that the energy required to excite an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) is high. Consequently, the wavelength of light absorbed is shorter and often falls below 200 nm, which is in the ultraviolet (UV) region of the electromagnetic spectrum.