UV-VIS spectra:

The UV-VIS spectra arise as a consequence of the interaction of the radiation with absorbing species causing a transition amongst their electronic energy levels. In case of gaseous samples, the absorbing species the spectrum consists of a number of closely spaced discreet lines that resemble a band and the resulting spectrum is called a band spectrum.

In the solution phase the energies of the quantum states of the molecules are somewhat spread out due to constant collisions with the surrounding solvent molecules. Therefore the sample absorbs photons spread over a range of wavelength and the spectrum acquires the shape of a smooth and continuous absorption peak.