Difference between a Continuous spectrum and Line spectrum

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Difference between a continuous spectrum and line spectrum

When light passes through an object, it forms what is known as a spectrum. This spectrum forms either a separated range of colors or waves and the properties of this separation will determine the kind of spectrum that forms. In addition, the way in which light passes through and separates after passing through an object can also reveal a lot about the physical properties of the materials from which the spectrum is being produced. A spectrum is therefore defined as a set of wavelengths that has been either emitted or absorbed from any form of object, substance, atom or molecule [1]. Common examples of spectra include micro waves, rainbows, ultraviolet radiation and x-ray waves and these can be derived from various different kinds of objects like prisms and dark solids [2]. There are three main types of spectra that are commonly produced. These are the emission line spectra, absorption line spectra and continuous spectra. The properties of continuous and line spectra are commonly confused because of their separation properties however they differ based on factors of temperature and regions where photon energy is either absorbed or emitted [5]. Below is an overview of the major differences between continuous and line spectra.

What are spectra?

Continuous spectra are characteristic of dense gases, solids and other substances which radiate heat away either through light production or at high temperatures. The light emission occurs over a wide range of wavelengths making the spectrum appear smooth and continuous [1]. A common example of a continuous spectrum is a rainbow as this spectrum goes all the way from red to violet with the colors fading into each other and not leaving any gaps in between. A beam of white light is also known to contain this kind of spectrum as when it is passes through a prism, it undergoes dispersion to produce the same continuous rainbow of colors [2]. Similarly when a black object is heated to glowing, it gives off radiation in the form of a continuous spectrum. Line spectra on the other hand can be defined as an electromagnetic spectrum that consists of discrete intermittent lines and occur either in the form of absorption spectra or emission spectra. As electromagnetic radiation passes through an object, some wavelengths will either be absorbed by the elements of the object's material or released in the form of excess energy, leading to the formation of lines in the spectrum. The lines generated will show up separate distinct gaps that are isolated in any particular spectrum, either as dark adsorption lines against a bright background or bright emission lines against a dark background [3].

How are continuous spectra and line spectra produced?

Spectra are commonly produced using a single source of light whose dispersion in turn results in the formation of a continuous spectrum. Generally when gas is exposed to high pressures, they will produce continuous spectra but under lower pressures, either absorption or emission spectra are produced. The same gas can produce an absorption spectra if the gas is cold or an emission spectra if the gas is produced together with heat [2]. If an electron is present in an excited energy state then there is a possibility of it returning to a lower energy level, thereby losing this energy. The difference in energy levels which the electron moves between is also equal to the amount of energy lost causing this energy to be released as a photon. In addition since different energy levels have different values, each transition and loss of energy will produce a photon with a different frequency and color. This therefore causes line emission spectra to be formed. On the contrary, continuous spectra are usually produced by shared electrons. These electrons are shared between atoms with a range of different frequencies [3]. Thus it can be said that a continuous spectrum is actually generated from putting both the absorption and emission spectra together.

Wavelengths and gaps

Continuous spectra contain all the wavelengths within a given range which in turn means there are no observable gaps in the spectra. This is clearly seen with rainbows as the seven colors fade smoothly into each other without any breaks or gaps. On the other hand, line spectra contain only a few wavelengths and are generated either by emission or absorption of energy thereby resulting in the production of visible gaps or lines within the spectrum. This is also an indication of larger gaps within a particular range of wavelengths.

Differences between continuous and line spectra

While it is easy to confuse the properties of continuous and line spectra, visual observations will clearly reveal some of the biggest differences between the two, in that continuous spectra have no gaps while line spectra have the formation of lines or dark gaps within the spectrum that is formed. Another big difference between the two is that line spectra can be recognized either by emission or absorption processes while continuous spectra can be produced by overlaying the emission and absorption spectra of the same element together. In addition, continuous spectra span entire wavelengths while line spectra only contain a few wavelengths.

Examples of continuous and line spectra

Common examples of objects emitting continuous spectra include white light dispersing through a prism, incandescent light bulbs, rainbows, stove burners, flames, stars and even the human body. Line spectra on the other hand can be present in one of two forms: bright lines that appear on a dark background or darker lines that appear on a bright background. Gases exposed to high pressure or which are colder will produce line spectra.

Summary of differences between a continuous spectrum and line spectrum

Continuous spectra Line spectra
Definition A spectrum that contains all wavelengths over a wide range An absorption or emission spectrum consisting of separated and isolated lines across a range of frequencies
Gaps There are no observable gaps There are huge gaps between the lines
Wavelength They contain all the wavelengths of a given range They contain a few wavelengths
Examples Radiation from rainbows or black bodies Hydrogen emission and adsorption spectra

Author: Shalinee Naidoo

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