Difference Between Continuous Spectrum and Line Spectrum

The key difference between continuous spectrum and line spectrum is that the continuous spectrum contains all the wavelengths in a given range whereas the line spectrum contains only a few wavelengths.

There are mainly two types of spectra as continuous spectrum and line spectrum. Line spectrum can create an absorption spectrum or an emission spectrum. Absorption and emission spectra of a species helps to identify those species and provide a lot of information about them.

CONTENTS

1. Overview and Key Difference
2. What is Continuous Spectrum
3. What is Line Spectrum
4. Side by Side Comparison – Continuous Spectrum vs Line Spectrum in Tabular Form
5. Summary

What is Continuous Spectrum?

When absorption and emission spectra of a species are put together, they form continuous spectrum. An absorption spectrum is a plot drawn between absorbance and wavelength. Sometimes instead of wavelength, we can also use frequency or wave number in the x-axis. Log absorption value or the transmission value is also useful for the y-axis in some occasions. The absorption spectrum is characteristic for a given molecule or an atom. Therefore, we can use it in identifying or confirming the identity of a particular species.

Thus, if all the wavelengths are present within a given limit, that is a continuous spectrum. For example, the rainbow has all seven colours, and it is a continuous spectrum. Continuous spectrum forms when hot objects like stars, moons emit electromagnetic radiations at all the wavelengths.

What is Line Spectrum?

As the name says, line spectrum has only a few lines. In other words, they have few wavelengths. For example, a coloured compound is visible to our eyes in that particular colour because it absorbs light from the visible range. Actually, it absorbs the complementary colour of the colour we see. For example, we see an object as green because it absorbs purple light from the visible range. Thus, purple is the complementary colour 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. Therefore, when we record the spectrum, it consists of a number of very narrow absorption lines. And this is an absorption line spectrum. It is characteristic of a type of atom. The atoms use absorbed energy to excite ground electrons to upper levels in the atom. Since the energy difference is discrete 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 is a spectrum having all the wavelengths within a given limit whereas line spectrum is a spectrum having some lines of wavelengths within a given limit. Thus, continuous spectrum and line spectrum differ from each other according to the presence or absence of lines in the spectrum. Therefore, we can consider this as the key difference between continuous spectrum and line spectrum. These lines occur in line spectrum because it contains only a few wavelengths while continuous spectrum contains all the wavelengths in a given range.

When considering the formation of each spectrum, we can find another significant difference between continuous spectrum and line spectrum. That is, in the formation of a continuous spectrum, both absorption and emission spectra of one species are put together whereas either absorption or emission spectrum generates the line spectrum.

Summary – Continuous Spectrum vs Line Spectrum

Continuous spectrum and line spectrum are two types of absorption and emission spectra. The key difference between continuous spectrum and line spectrum is that the continuous spectrum contains all the wavelengths in a given range whereas the line spectrum contains only a few wavelengths.

Reference:

1. Libretexts. “6.3: Line Spectra and the Bohr Model.” Chemistry LibreTexts, Libretexts, 25 July 2018. Available here   

Image Courtesy:

1.”Spectrum-sRGB”By Phrood – Own work, (Public Domain) via Commons Wikimedia  
2.”Sodium and calcium spectra”By NASA (Public Domain) via Commons Wikimedia