Continuous Spectrum vs Line Spectrum
There are mainly two types of spectra as continuous and line spectra. Line spectra can be an absorption spectrum or an emission spectrum. Absorption and emission spectra of a species help to identify those species and provide a lot of information about them. When absorption and emission spectra of a species are put together, they form the continuous spectrum. An absorption spectrum is a plot drawn between absorbance and wavelength. Sometimes instead of wavelength, frequency or wave number can also be used in the x axis. Log absorption value or the transmission value is also used for the y axis in some occasions. Absorption spectrum is characteristic for a given molecule or an atom. Therefore, it can be used in identifying or confirming the identity of a particular species.
Atoms, ions, and molecules can be excited to higher energy levels by giving energy. The lifetime of an excited state is generally short. Therefore, these excited species have to release the absorbed energy and come back to the ground state. This is known as relaxation. The release of energy may take place as electromagnetic radiation, heat or as both types. The plot of released energy versus wavelength is known as the emission spectrum. Each element has a unique emission spectrum as they have a unique absorption spectrum. So radiation from a source can be characterized by emission spectra.
If all the wavelengths are present within a given limit, that is a continuous spectrum. For example, rainbow has the all seven colors and it is a continuous spectrum. Continuous spectra are formed when hot objects like stars, moons emit electromagnetic radiations at all the wavelengths.
As the name says, line spectrum has only few lines. In other words they have few wavelengths. For example, a colored compound is visible to our eyes in that particular color because it absorbs light from the visible range. Actually, it absorbs the complementary color of the color we see. For example, we see an object as green because it absorbs purple light from the visible range. Thus, purple is the complementary color of green. Likewise, atoms or molecules also absorb certain wavelengths from the electromagnetic radiation (these wavelengths are not necessarily to be in the visible range). When a beam of electromagnetic radiation passes through a sample containing gaseous atoms, only some wavelengths are absorbed by the atoms. So when the spectrum is recorded, it consists of a number of very narrow absorption lines. And this is an absorption line spectrum. It is characteristic to a type of atom. The absorbed energy is used to excite ground electrons to upper levels in the atom. Since the energy difference is discreet and constant, the same kind of atoms will always absorb the same wavelengths from the given radiation. When this excited electron is coming back to the ground level, it emits the absorbed radiation and it will form an emission line spectrum.
What is the difference between Continuous Spectrum and Line Spectrum?
• Continuous spectrum contains all the wavelengths in a given range whereas line spectrum contains only few wavelengths.
• Line spectra can be generated in absorption and emission. When both absorption and emission spectra of one species is put together, it forms a continuous spectrum.