What is Spectrum ?

Definition:

A visual display dispersion of components of white light when it is passed through a prism is called spectrum. The study of spectrum is called spectroscopy. The instrument used to analyze spectrum is called spectrometer.

Explanation:

Ordinary white light consists of radiations of all wavelengths. After passing through the prism, white light is splitted up into radiations of different wavelengths. The colours of visible spectrum are violet, indigo, blue, green, yellow, orange, red and their wavelengths ranges from 400 nm to 75.0 nm.

These radiations are called visible light radiations because our eyes can detect only these radiations. In addition to visible region of spectrum, there are seven other regions. Ultra violet, x-rays, γ-rays and cosmic rays are towards the lower wavelength end of the spectrum and they possess photons with greater energy.

On the other side of visible region, there lies infra-red, microwaves and radio waves. The arrangement of all types of electromagnetic radiations in the decreasing order of their wavelengths or increasing order of their frequencies is called electromagnetic spectrum and is shown in figure above.

Types of spectrum:

Spectrum is of two types:

1. Continuous Spectrum
2. Line Spectrum

1. Continuous spectrum:

In this type of spectrum, the boundary line between the colours cannot be marked. The colours diffuse into each other. One colour merges into another without any dark space.

The best example of continuous spectrum is rainbow. It is obtained from the light emitted by the sun or incandescent (electric light) solids. It is the characteristic of matter in bulk.

2. Line spectrum (atomic spectrum):

When an element or its compound is volatilized on a flame and the light emitted is seen through a spectrometer, we see distinct lines separated by dark space. This type of spectrum is called tine spectrum or atomic spectrum.

This is characteristic of atom. Each element has its own line spectrum. The number of lines and distance between them depend upon the element volatilized.

Examples:

Line spectrum of sodium contains two yellow coloured lines separated by a definite distance. Similarly the spectrum of hydrogen consists of a number of lines of different colours having, different distance between them.

It has been observed that distance between lines decreases becomes continuous after with the decrease in wavelength and a certain value of wavelength. Atomic spectra or line spectrum are of two types:

1. Atomic emission spectrum
2. Atomic absorption spectrum

1. Atomic emission spectrum:

It is produced by the radiation emitted by excited atoms. The atoms can be excited by:

• Heating the substance strongly.
• Passing electric discharge through a gas at very low pressure.
• Passing electric current through a thin metal filament.

 

 

When, radiations emitted by the excited substance are analyzed by a spectrograph (instrument used to record a spectrum) a discontinuous spectra consisting of a series of bright lines against dark background is obtained. This is called atomic emission spectrum. Emission line spectrum is a finger print of atoms because:

• Each element has its characteristic spectrum just as each individual has its characteristic finger print.
• Each line in the spectrum corresponds to a particular wavelength or frequency.

2. Atomic absorption spectrum:

When a beam of white light (light consisting of all wavelengths) is passed through vapours of an element or solution of a substance, it may absorb certain wavelength while the rest of wavelengths pass through the prism.

The spectrum of this radiation is called atomic absorption spectrum. The way lengths of radiation that have been absorbed by the element appear as dark lines and the background is bright.

It is interesting to note that the wavelengths of the dark line in the absorption spectrum of a substance are the same as those of the wavelength bright lines in the emission spectrum of the same substance.

In emission spectrum these lines appear bright because the corresponding wavelengths are being emitted by the element, whereas they appear dark in the atomic absorption spectrum because the wavelengths are being absorbed by the element.