White light is composed of continuous wavelengths of light in the visible region (400-700nm). Here we simplify its representation with discrete colors arranged in descending order wavelength (i.e., increasing energy). Polarization does not affect absorption and is not represented; complementary colors are denoted with yellow dotted lines.

"Beam source" generally indicates a broad spectrum light, e.g., from black-body radiation. Lasers are not within this scope as they are monochromatic (single wavelength).

Molecules have quantized energy levels, and photons have quantized energy. If the incoming photon has exactly the matching energy for the promotion of the molecule, the molecule would absorb the photon, and change state from the ground state to an excited state. The specific physical change depends on the radiation.

Light in the visible region induces electronic excitation (rearrangement of electron clouds).

This applies only for visible light - the removal of a color results in the beam appearing as its complementary color. Here, the removal of green within the red-green pair gives the transmitted beam a red color.

Note that absorptions always has a non-zero line-width despite the quantized nature of light and matter. This is discussed in the article on line broadening.

A spectrum is effectively a (continuous) histogram describing the composition of light. A spectrum can be represented as a transmission spectrum (shown in gray) where the y-axis shows the photon count of transmitted light, or an absorption spectrum (magenta) where the attenuation is plotted. The former is commonly used in infrared/microwave spectroscopy, and the latter in UV-Vis and NMR.

Historically a detector is an optical device where the beam is split (by a diffraction grating or prism) and captured on a photographic plate. Modern spectroscopy usually uses a CCD array in conjunction with digital electronics.

In the laboratory, this is commonly a sample containing an analyte dissolved in a solvent. Spectroscopy is a general technique - an example on the astronomical scale would involve a star as the "beam source", a planet as the "sample", and a telescope-CCD array as the detector, with interstellar space between each. The concepts and principles remain the same.

Despite the quantized nature of light and matter, absorptions are broad and never of zero line-width. This phenomenon is explained in the article on line broadening.