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Light is electromagnetic radiation that the human eye can detect. In the 1870s, Maxwell explained that electric fields coupled with magnetic fields produce electromagnetic waves. These waves didn’t require any medium for propagation. Light exhibits both particle and wave behavior. Light exhibits a photoelectric effect as proof of its particle nature. Light as a wave was confirmed by Young’s double-slit experiment.
Electromagnetic radiations can be described by the following three factors: frequency, wavelength, and energy.
The number of times a crest passes through a point per second is called frequency (s-1 or Hz).
The distance between 2 crests is known as wavelength (m).
In terms of energy, a unit called electron volts, eV is used. It is defined as the amount of energy required to move an electron through the potential of 1 V.
Properties of light
Visible light is a small part of the electromagnetic spectrum, which is basically all the forms of electromagnetic radiation. Other examples of electromagnetic radiations are ultraviolet rays, radio waves, infrared radiations, microwaves, etc. These waves do not require any medium to propagate. These consist of mutually perpendicular electric and magnetic fields, which are, in turn, perpendicular to the direction of wave propagation. They all move with the same velocity in a vacuum, c = 3.00 x 108 m/s. The relation can be characterised by the frequency, v , and wavelength, λ, by the relation c = v λ.
The following relation can give the energy of electromagnetic radiations:
E = hv = h(c/λ), where h is Planck’s constant.
Based on frequency or energy, we can arrange the seven electromagnetic radiations in the form of a spectrum:
Radio waves < microwaves < infrared radiations < visible light < ultraviolet radiations < X-rays < γ- rays.
In terms of frequency and energy, radio waves have minimum energy and frequency, whereas γ- rays have the highest energy and frequency.
In terms of wavelength, radio waves have the largest wavelengths, whereas γ-rays have the shortest wavelengths. So, the spectrum will be as follows:
Radio waves > microwaves > infrared radiations > visible light > ultraviolet radiations > X-rays > γ- rays.
Spectroscopy
Spectroscopy refers to many techniques that employ radiation to obtain information about the structure and properties of matter. The basic principle of every spectroscopy technique is shining a beam of electromagnetic radiation onto a sample and observing its response to that stimulus. The response is then recorded as a function of the wavelength of radiation. Such a plot is referred to as a spectrum.
Light is made of different wavelengths and frequencies. Every element in the periodic table has a unique spectrum of light depending on the frequency of light which it emits or absorbs.
So, we can broadly classify spectra into two types: emission spectra and absorption spectra.
- Emission spectra: On excitation, i.e., on heating, passing an electric discharge, etc., some substances emit light. It gives bright (coloured) lines on a dark background. It may be continuous (when light emitted by the source is white) or maybe discontinuous (if light emitted is coloured). It is also referred to as the line spectrum. Examples include spectra of white light.
- Absorption spectra: When atoms absorb energy to reach higher energy levels, absorption spectra are produced. Dark lines on the bright background can be seen. These are always discontinuous. Examples include UV — visible absorption spectroscopy and NMR spectroscopy.
Sources of lights
We saw that for any type of spectroscopy, we have to shine a beam of electromagnetic radiation on the sample. For that, we need some source of light. Light sources can be natural, like the sun and stars. They can also be man-made, like a lamp.
Basically, we categorise light sources into two parts: Coherent sources and incoherent sources. This classification is done based on phase differences and frequencies of the two waves.
Coherent sources of light
We call two sources coherent if they produce two waves with the same frequency, waveform, and a constant phase difference. These waves can interfere constructively to give bright fringes. Examples include laser lights.
Incoherent sources of light
These are exactly opposite to coherent sources. We call two sources incoherent sources if they produce 2 waves that are not in phase and have different frequencies and waveforms. Examples of incoherent sources are tungsten filament lamps, sodium lamps, etc. So we can say that the meaning of incoherent source is a source that produces waves that are out of phase with each other. So these waves can’t interfere constructively.
Conclusion
Light is electromagnetic radiation. It has a dual nature, which implies that it behaves both as a particle and a wave. Electromagnetic radiations are those that do not need any medium to propagate and consist of mutually perpendicular electric and magnetic fields, which are perpendicular to the direction of propagation of waves. Visible light is also a part of the electromagnetic spectrum. These rays have numerous uses. One such use is in spectroscopy techniques. The basic principle of spectroscopy is based on the irradiation of a beam of light on the sample to receive a response. That response is recorded as a function of wavelength. Spectroscopy can be classified as absorption or emission spectroscopy. When light is emitted on excitation, emission spectra are produced. They show bright lines on a dark background. An absorption spectrum is produced when light is absorbed to reach a higher level. They show dark lines on a bright background.
Sources of lights used can be coherent sources or incoherent sources. Coherent sources produce waves of the same frequency and have constant phase differences. On the other hand, out of phase and having different frequencies waves are produced by incoherent sources. Examples include laser lights and sodium lamps, respectively.
