Different Types of Spectra
Any light source can produce a spectrum by scattering its light. The causes of various types of spectra can be derived from atomic theory. The spectra obtained from different light sources are mainly divided into two categories: 1. Emission spectrum, and 2. Absorption spectrum.
Emission Spectrum
Any object can be excited so that it emits light. The nature of this light depends on the temperature or material of the object. The spectrum that this light produces in the spectroscope is called the emission spectrum.
An object can be excited in many ways. For example, by increasing the temperature of the material, reducing the material to a gaseous state and placing it under low-pressure by-passing electricity through them, etc.
A spectroscope is an optical instrument. With it, spectra are generated and analyzed.
Emission spectra are again divided into three types according to structure: 1. Continuous spectrum, 2. Line spectrum, and 3. Band spectrum.
Continuous Spectrum
An emission spectrum in which all seven colors from red to violet are arranged continuously in the form of bright bands is called a continuous spectrum. There is no gap between this spectrum, i.e., between red and violet. In other words, this spectrum is not divided into multiple parts by any black (dark) line or band. Hence it is called a continuous spectrum. Obviously, all wavelengths of light are present in this spectrum.
Source: Such spectra are emitted from incandescent solids and liquids due to high temperatures. Like the incandescent filament of an electric lamp, white hot molten metal, etc. When a piece of iron is heated very much, it becomes red. This state is called a hot state. If the piece is heated further, it turns white. This state is called a white hot state. White hot metal can be in a solid state or in a liquid state. The filament of an incandescent electric lamp is white-hot, and the filament of an electric heater is red-hot. Besides, this type of spectrum is also emitted from incandescent gaseous materials kept at high pressure. For example, the solar spectrum can be called a continuous spectrum.
Characteristics: The yellow part of the continuous spectrum is the highest in brightness, and the brightness decreases gradually towards the two ends of the yellow, i.e., the red and violet ends. The temperature of the source is roughly known from the continuous spectrum, but no information is known about the chemical composition of the source. Because continuous spectra produced by substances of different chemical compositions look almost identical.
Line Spectrum
An emission spectrum in which several fine parallel lines of different colours are seen separately is called a line spectrum. Between these spectral lines are black (dark) bands. That is, except for the lines, the rest of the spectrum is dark. Hence its name, line spectrum. Obviously, not all wavelengths of light are present in this spectrum.
Source: When a gaseous substance glows, the spectrum of light emitted from its atoms is called the line spectrum. For example, a line spectrum is obtained from the light emitted by passing an elemental gas under low pressure in a discharge tube, from a vapor lamp, or when a salt is exposed to a Bunsen flame.
Characteristics: Light emitted from different types of atoms produces different types of line spectra. So, it can be said that the line spectrum is characteristic of atoms or elemental substances. So, any elemental substance can be identified by looking at this spectrum. The color and position of the lines in the line spectrum of each elemental substance are specific. For example, two very close yellow lines appear in the line spectrum of sodium. Experiments have shown that no element other than sodium produces such a yellow line. Again, the line spectrum of hydrogen gas does not produce any other atoms. It is therefore possible to detect any element by reviewing the line spectrum.
Analyzing line spectra provides a lot of information about the structure of atoms. By analyzing the spectrum of a star, it is known what elements are present in that star. The number of elements can often be estimated by looking at the brightness of the spectrum lines.
Line spectra are characteristic of atoms or elemental substances.
Band Spectrum
An emission spectrum in which several bright light bands of different colors are seen separately is called a band spectrum. (When a line is thickened, it takes on the shape of a band.) Between the bright bands of this spectrum are black (dark) bands. That is, except for the bright bands, the rest of the spectrum is dark. Hence its name, band spectrum. Obviously, not all wavelengths of light are present in this spectrum.
Source: When a gaseous substance glows, the spectrum of light emitted from its molecules is a band spectrum. For example, the band spectrum is obtained from the light emitted when electrolysis is performed at low voltage through gases such as oxygen, nitrogen, etc. at low pressure in the tube. Similarly, band spectra are also obtained from compound molecules like nitric oxide, cyanogen, etc.
Characteristics: One side of each band is very bright and clearly demarcated. This side of the band is called the band head. Going from one direction to the other, the brightness of the band gradually decreases and merges with the dark area. As a result, the boundary is unclear. This side of the band is called the band tail. A close look at the bands reveals that each band has numerous fine lines very close to each other. The distance between two lines located side by side is extremely small towards the top of the band. This distance increases gradually as the band moves from the top to the tail of the ribbon.
A band spectrum is a spectrum of light emitted from molecules of an element or compound. Therefore, by analyzing the band spectrum, a lot of information about the structure of the molecule can be obtained.
Absorption Spectrum
When white light passes through a transparent material, one or more colors of white light can be absorbed by the transparent material. The light of the rest of the colors emitted from the transparent material creates a spectrum in the spectroscope called the absorption spectrum. Each letter in the spectrum has a specific position. Therefore, the black (dark) region appears at the position of the spectrum where the color is absorbed by the transparent material. Absorption spectra are divided into two parts according to structure: 1. line absorption spectrum, and 2. band absorption spectrum.
Line Absorption Spectrum
An absorption spectrum in which a few fine black lines are seen in isolation is called a line absorption spectrum.
Source: An atom of a gas or vapor forms a line absorption spectrum with light emitted by an external light source passing through it at low pressure and temperature. Like sodium vapor, iron vapor, hydrogen, etc. This spectrum is created because the gas absorbs light of certain wavelengths. It has been found that the gas absorbs only those wavelengths of light that the gas emits when heated. As a result, the spectrum of light passing through the gas looks like a continuous spectrum, but the spectrum shows black lines for absorbed wavelengths.
Placing heated sodium vapor in the path of white light emitted from a white hot carbon arc lamp produces two black lines in the continuous spectrum of white light. This is the line absorption spectrum of sodium. The two yellow lines in the sodium vapor emission spectrum are exactly where the two black lines in the absorption spectrum of sodium are found.
Characteristics: The black lines are parallel to each other, and there are cultured areas between two consecutive lines. Like the emission line spectrum, the position of the black lines in the absorption spectrum also depends on the nature of the gas. Therefore, the absorbing material can be identified by looking at the position of the black line in the line absorption spectrum. That is, the line absorption spectrum reveals the properties of the atoms of the absorbing material.
Band Absorption Spectrum
An absorption spectrum in which several black broad bands are seen separately is called a band absorption spectrum. Black bands are called absorption bands.
Source: Typically, diatomic and polyatomic gases or vapors form band absorption spectra with light emitted by an external light source passing through them at low pressure and temperature (lower temperature than the external light source). Gases like O2, N2, CO2, etc.
Characteristics: The absorption band consists of numerous black absorption lines closely spaced. The coloured bands found in the emission spectrum of a substance are replaced by black bands in the absorption spectrum. Because when the gas glows in its molecular state, the band of colors it emits, the molecules of the gas absorb those colors from the white light. As a result, black bands are seen in the absorption spectrum of the gas, where the bright bands were formed in the emission spectrum. Thus, the band absorption spectrum reveals the properties of the molecules of the absorbing material.