Definition of Spectrum
Before we discuss the emission spectrum definition, let us address the questions - what is a spectrum in chemistry and what is a spectrum in physics. Whether it is physics or chemistry, the spectrum definition is the same - when white light is passed through a prism or any other dispersing substance, the white light splits into a series of coloured bands or lines known as a spectrum. The different constituent wavelengths of white light are arranged in the spectrum in a specific order, starting with the longest wavelength (red) and shading through to the shortest (violet).
(Image will be Uploaded Soon)
What is Emission Spectrum?
So, what is the emission spectrum definition in physics and chemistry? An emission spectrum is the range or array of wavelengths (spectra) obtained when the light emitted by a substance is passed through a prism and examined directly with a spectroscope.
Now let's define the line emission spectrum: a spectroscope splits the emitted light into different wavelengths and gives a discontinuous spectrum in the form of discrete lines known as a line spectrum. An example of an emission spectrum is when copper is heated on a flame, and the flame gets green color.
Production of Emission Spectrum
When an atom or molecule absorbs energy, the electrons are excited to a higher energy level. When the electron falls back to the lower energy level, light is emitted, which has the energy equivalent to the higher and the lower states’ energy difference. Due to the availability of multiple states of energy, an electron can undergo numerous transitions, each giving rise to a unique wavelength that comprises the emission spectrum.
Atomic Spectra
We know that when elements or their compounds are heated, they release energy in the form of light, which gives rise to a line spectrum. However, when atoms in their elemental form are heated or excited, the line spectra that originate are known as the atomic spectra.
Absorption Spectrum
When electromagnetic radiation passes through a material, a part of the electromagnetic radiation may be absorbed. In that case, when the remaining radiation is passed through a prism, a spectrum is obtained with a gap in it, called an absorption spectrum. The absorption spectrum is characteristic of a particular element or compound and does not change with varying concentrations.
(Image will be Uploaded Soon)
Hydrogen Emission Spectrum
When the hydrogen atom gets energy from outside, its electron goes from the lowest energy level to some higher energy level. But it returns from there, within 10-8 seconds. To the lowest energy level directly or via other lower energy levels. While returning, the electrons emit light.
Suppose two energy levels of the hydrogen atom, n1, and n2, have energies E1 and E2, respectively. If the electron's transition takes place from the higher energy level n2 to the lower energy level n1, it will emit a photon of light of energy (E2 - E1). The frequency 'ν' of the emitted light is given as:
\[ v = R ((\frac{1}{n_{1}^{2}} ) - (\frac{1}{n_{2}^{2}})) \] where ‘R’ is the Rydberg constant.
Hydrogen Transitions
Lyman series: An electron on returning from some higher energy level to the first energy level (that is, n1 = 1 and n2 = 2, 3, 4, etc.), then the emitted series of spectral lines are obtained in the ultraviolet region.
Balmer series: An electron on returning from a higher energy level to the second energy level (that is, n1 = 2 and n2 = 3, 4, 5, etc.), the emitted spectral lines are obtained in the visible region.
Paschen series: An electron on returning from some higher energy level to the third energy level (that is, n1 =3 and n2 = 4, 5, 6, etc.), then the emitted lines are obtained in the infrared region of the spectrum.
Brackett series: An electron on returning from some higher energy level to the fourth energy level (that is, n1 = 4 and n2 = 5, 6, 7, etc.), the emitted lines are obtained in the infrared region of the spectrum.
Pfund series: An electron on returning from a higher energy level to the fifth energy level (that is, n1 = 5 and n2 = 6, 7, 8, etc.), the emitted lines are obtained in the infrared region of the spectrum.
A spectrum is like a graph that depicts the intensity of light emitted across a wide range of energies giving a band of colors such as an example of a rainbow. The emission spectra of an element is the spectrum of radiation emitted due to an atom or molecule absorbing energies and transitioning from a high energy state to a lower energy state.
Atoms, molecules or ions that absorb radiation are known to be in an excited state. When the radiations emitted by different substances which are produced in several regions of magnetic spectrum while jumping between different energy levels are analyzed, the spectrum obtained is a well-defined line, corresponding to a specific frequency or wavelength. This spectrum produced by electrons in the excited state of atoms or molecules is termed as the emission spectrum.
Examples of Emission Spectra
When the light passes through a prism, it breaks down into a spectrum of colored well-defined lines with different wavelength characteristics.
Types of Emission Spectra
Line Spectrum
Continuous spectrum
Band spectrum
Line Spectrum
The line spectrum otherwise known as the atomic spectrum is obtained by analyzing the radiation emitted by passing an electric discharge through hydrogen gas at low pressure. Such a spectrum consisting of lines of definite frequencies is called line spectrum or discontinuous spectrum.
Continuous Spectrum
It is a type of spectrum that is made up of continuous luminous bands of all colors from violet to red. Continuous spectrums are only affected by the temperature of the source and not by any other characteristics. Continuous spectra are produced by incandescent solids, liquids, and electric filament lamps etc.
Band Emission
A spectrum is made up of groups or bands bright at one end and dull at another of closely spaced lines. When the light emitted or absorbed by molecules is viewed through a high power resolving spectroscope with a small dispersion, the spectrum appears to be made up of a very wide range of fine lines known as bands which have a number of bright bands that have a sharp edge at one end but fade out at the other.
Uses of Emission Spectra
1. The emission spectrum is different for each element in the periodic table and can be used to determine the material composition. One example is astronomical spectroscopy which includes identifying the composition of stars by analyzing the received light.
2. All hot material will emit light. Examples include the stove element in the kitchen, the metal filament in a lightbulb, and the sun as well. Scientists observed this phenomenon in their laboratories but were not able to explain how and why it occurs.
Hydrogen Emission Spectrum
We all know that electrons absorb energy and get excited, they jump from a lower energy level to a higher energy level, and when they return to their original states they emit radiation. This same phenomenon explains the emission spectrum through hydrogen as well, which is known as the hydrogen emission spectrum. The spectrum is made up of a large number of well-defined lines appearing in different regions as per their wavelengths such as some present in visible regions, while others are in ultraviolet and infra-red regions.
FAQs on Emission Spectrum
1. What is an emission spectrum in simple terms?
An emission spectrum is the unique pattern of light frequencies, or colours, that an element gives off when it is energised. Think of it as a chemical fingerprint. When electrons in an atom get excited and then fall back to their normal state, they release this energy as light, creating a series of bright lines that are specific to that element.
2. What is the main difference between an emission spectrum and an absorption spectrum?
The main difference is in how they are made and what they look like.
- An emission spectrum is produced when an element releases light. It appears as a series of bright, coloured lines on a dark background.
- An absorption spectrum is created when light passes through an element, which absorbs certain frequencies. It looks like a continuous rainbow with dark lines where the light was absorbed.
3. What are the main types of emission spectra?
There are three main types of emission spectra:
- Continuous Spectrum: This is an unbroken band of colours, like a rainbow, produced by hot, dense objects such as the filament of a light bulb.
- Line Spectrum: This consists of distinct, separate bright lines of specific colours. It is produced by excited atoms in a gas and is unique to each element.
- Band Spectrum: This consists of groups of lines that are very close together, forming bands. It is characteristic of molecules rather than individual atoms.
4. How is an emission spectrum actually produced?
An emission spectrum is produced through a simple process at the atomic level. First, an atom absorbs energy from heat or electricity, causing one of its electrons to jump to a higher, unstable energy level. This state is temporary, so the electron quickly falls back to its original level. As it falls, it releases the extra energy as a particle of light called a photon, which contributes to the unique emission spectrum.
5. Why is the emission spectrum of every element unique?
An element's emission spectrum is unique because every element has a distinct arrangement of electrons and quantised energy levels. The energy gaps between these levels are fixed and different for each element. Therefore, when an electron falls from a higher to a lower level, it always emits a photon with an exact amount of energy, creating a pattern of lines that no other element can replicate.
6. How do astronomers use emission spectra to study distant stars?
Astronomers use emission spectra to determine the chemical composition of stars. They analyse starlight with an instrument called a spectrometer, which splits the light into a spectrum. The pattern of bright lines they see is then matched to the known emission spectra of elements. This allows them to identify which elements are present in the star, even from millions of light-years away.
7. What are the different spectral series in the hydrogen emission spectrum?
The hydrogen emission spectrum is organised into several series, named after their discoverers. Each series is formed when electrons fall to a specific final energy level. The main series are:
- Lyman Series: Electrons fall to the 1st energy level (found in the UV region).
- Balmer Series: Electrons fall to the 2nd energy level (partially visible light).
- Paschen Series: Electrons fall to the 3rd energy level (infrared region).
- Brackett Series: Electrons fall to the 4th energy level (infrared region).
- Pfund Series: Electrons fall to the 5th energy level (infrared region).
