The electromagnetic spectrum is the range of frequencies EM radiations along with their associated wavelengths and photon energies. It consists of Gamma-rays, X-rays, ultraviolet rays, infrared rays, radio waves and microwaves. Electromagnetic radiations have a wide range of frequencies, wavelengths and photon energy levels. These waves travel at the speed of light in vacuum.
The plural form of spectrum is a spectra. Spectrum is widely used in the field of optics and many more fields. Spectrum features a wide range of wavelengths of different frequency radiations. A rainbow is a spectrum that constitutes different wavelengths of light. The spectrum of light from the rainbow is commonly referred to as VIBGYOR. The prism is also an ideal example to describe the spectrum of radiation. When the white light passes through the prism, it gets separated into different wavelengths of light known as the spectrum.
The instrument used to separate the radiations of different wavelengths is known as Spectroscope or Spectrograph. A spectrometer is a scientific device that aids to separate and measure spectral components of a physical phenomenon. Branch of science that deals with the study of the spectrum are known as spectroscopy.
A spectrograph features a prism or diffraction grating that is used to disperse light. The emerging light after the dispersion from the prism is examined using a photographic film.
Classification of Spectra
Spectra is classified into two types:
- Emission spectra
- Absorption spectra
The main difference between emission and absorption spectra is that an emission spectrum has different coloured lines in the spectrum, whereas an absorption spectrum has dark-coloured lines in the spectrum.
What is emission spectra?
The spectrum formed by the radiation emitted by electrons in the excited molecules or atoms is known as the emission spectrum. The emission spectrum comprises frequencies of electromagnetic radiation due to the influence of chemical elements or chemical compounds emitted due to an atom or molecule, making the transition from a high energy state to a lower energy state. These excited electrons have to radiate energy to return to ground states from the excited state, which is unstable. The frequencies of these emitted light form the emission spectrum.
In general, an emission spectrum describes the wavelengths of the electromagnetic spectrum emitted by an energetic object. What this object is depends on the scientific discipline.
In chemistry, an emission spectrum refers to the range of wavelengths emitted by an atom or compound stimulated by either heat or electric current. An emission spectrum is unique to each element. The emission spectrum of burning fuel or other molecules may also be used to example its composition.
In astronomy, the emission spectrum generally refers to the spectrum of a star, nebula, or another body.
Emission spectra for atoms appear as a series of lines, because electrons fall from higher energy states to lower ones and emit energy as electromagnetic radiation.
How an Emission Spectrum Is Produced
When an atom or molecule absorbs energy, electrons are bumped into a higher energy state. When the electron drops to a lower energy state, a photon is released equal to the energy between the two states. There are multiple energy states available to an electron, so there are many possible transitions, leading to the numerous wavelengths that comprise the emission spectrum. Because each element has a unique emission spectrum, the spectrum obtained from any hot or energetic body may be used to analyze its composition.
What is Absorption Spectra?
When light from any source is passed through the solution or vapour, a pattern comprising dark lines is obtained. This pattern is analysed using the spectroscope. Depending on the nature of the chemical or element, certain radiation is absorbed by the chemical or element when passed through it.
The dark line pattern is seen exactly in the same place where coloured lines are seen in the emission spectrum. The spectrum thus obtained is known as the absorption spectrum.
In other words, this spectrum is constituted by the frequencies of light transmitted with dark bands when the electrons absorb energy in the ground state to reach higher energy states. This type of spectrum is produced when atoms absorb energy.
Absorption spectra are measured by varying the wavelength of the incident light using a monochromator and recording the intensity of transmitted light on a detector.
Emission vs Absorption Spectra: Key Difference
| Points of Comparison | Emission Spectra | Absorption Spectra |
| Description | Emission spectra is a spectrum which consisting of few wavelength with distinct spacing between them. | Absorption spectra is a spectrum which consisting wavelengths having dark lines between them. |
| Formation | Formed by radiation released by electrons transitioning from a high energy state to low energy state to attain stability. | Formed from the absorption of incident frequencies by the material. |
| Nature | Consists of lines of frequencies of emitted radiation in an empty black band. | Consists of black lines of absorbed frequencies in between the continuous spectrum of incident frequencies. |
| Atoms | An atom will have higher energy while exhibiting the emission spectrum. | An atom will have lower energy while exhibiting the absorption spectrum. |
| Nature | Emission spectra is a discontinues spectra. | Absorption spectra is a continues spectra. |
| Appication | Can determine the elements present in a given sample material. | Can determine the different types of compounds present in a mixture. |
| Use | The intensity of spectral lines gives the amount of an element present. | The intensity of spectral lines gives the number of elements present in the mixture. |
| Composition | Comprise coloured lines in the spectrum. | Comprise dark lines or gaps in the spectrum. |
Key takeaways
- Emission spectra involve electrons moving from lower to higher energy levels, which occurs when they take in energy. These excited electrons must then release, or emit, this energy to return to their ground states. The frequencies of this emitted light comprise their emission spectrum.
- Absorption spectra concern light frequencies of electrons that absorb energy. These electrons move from their ground states to higher energy states. The frequencies of this absorbed light comprise their absorption spectrum.
- Absorption spectroscopy is a spectroscopic technique that is used for measuring the absorption of radiation as it interacts with the sample.
- Emission spectra can emit all the colours in an electromagnetic spectrum, while the absorption spectrum can have a few colours missing due to the redirection of absorbed photons. The wavelengths of light absorbed help figure out the number of substances in the sample.