UV-Visible Spectrum
The diagram below shows a simple UV-visible absorption
spectrum for buta-1,3-diene. Absorbance (on the vertical axis)
is just a measure of the amount of light absorbed.
One can readily see what wavelengths of light are
absorbed (peaks), and what wavelengths of light are
transmitted (troughs). The higher the value, the more
of a particular wavelength is being absorbed.
The absorption peak at a value of 217 nm, is in the ultra-violet
region, and so there would be no visible sign of any light being
absorbed making buta-1,3-diene colourless. The wavelength
that corresponds to the highest absorption is usually referred
to as “lambda-max” (lmax).
The spectrum for the blue copper complex shows that the
complementary yellow light is absorbed.