Dual Behaviour of Electromagnetic Radiation: A blacksmith’s horseshoe appears red, whereas a higher-temperature object, such as the sun’s surface, appears yellow or white. When a drop of water hits a smooth water surface, it creates a series of waves that flow outward in a circular pattern. Electromagnetic radiation also behaves in a similar way to that of water ripples.

Originally, electromagnetic radiation was thought to solely have a wave character, and we could use this to explain phenomena like interference and diffraction. However, due to the wave nature of electromagnetic radiation, certain phenomena such as black body radiation and the photoelectric effect could not be explained. In this article, we will provide detailed information on the dual behaviour of electromagnetic radiation.

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Particle Nature of Electromagnetic Radiation

Electromagnetic waves exhibit a particle nature. This property of electromagnetic waves is demonstrated by blackbody radiation and the photoelectric effect.

Blackbody Radiation

An electric stove burner or a space heater filament on being heated glows into a dull red or orange, whereas an incandescent light bulb’s much hotter tungsten wire emits a yellowish light. All objects, when heated, produce electromagnetic radiation whose wavelength (and colour) are determined by the object’s temperature rather than its surface or composition. 

The intensity of radiation measures the amount of energy emitted per unit area. Below is a plot showing blackbody radiation strength as a function of wavelength for an object at various temperatures. It was observed that there is a dramatic fall in the intensity of light emitted at shorter wavelengths (mainly in the ultraviolet part of the spectrum). This dramatic fall is known as the ultraviolet catastrophe. The classical physics that assumed electromagnetic radiation to be continuous could not explain this catastrophe. 

However, in \(1900\), German physicist Max Planck \((1858–1947)\) proposed that the energy of electromagnetic waves is quantized rather than continuous, explaining the ultraviolet catastrophe. This indicates that only integral multiples of some smallest unit of energy, a quantum, could be obtained or lost.

Planck postulated that the equation could describe the energy of a particular quantum of radiant energy

\(\text {E}=\text {hv}\)

where the proportionality constant \(\mathrm{h}\) is called Planck’s constant \(6.626 \times 10^{-34} \mathrm{Js}\)

Photoelectric Effect

Electrons are discharged from the surface of certain metals when they are exposed to light. According to classical physics, the number of electrons emitted and their kinetic energy should be determined solely by the intensity of the light, not its frequency. However, it was found that each metal has a characteristic threshold frequency of light, below which no electrons are emitted regardless of the intensity of light.

Above the threshold frequency, the number of electrons emitted was found to be proportional to the intensity of the light; however, their kinetic energy was proportional to the frequency. This phenomenon was called the photoelectric effect (A phenomenon in which electrons are ejected from the surface of a metal that has been exposed to light).

Based on Planck’s concept, Einstein proposed that each metal has a specific electrostatic attraction for its electrons that must be overcome before an electron is emitted from its surface \(\left({\rm{E_{o}=h v_{o}}}\right)\).

If photons of light strike a metal surface with energy less than \(\text {E}_{\text {o}^{\prime}}\), then no electrons are emitted regardless of the intensity of the light.

If a photon strikes the metal with energy greater than \(\text {E}_{\text {o}^{\prime}}\) then part of this energy is used in overcoming the forces that hold the electron to the metal surface, and the extra energy appears as the kinetic energy of the ejected electron, which is given by the equation-

Kinetic energy of ejected electron \(=\text {E}-\text {E}_{\text {o}}=\text {h} \text {v}-\text {hv}_{\text {o}}=\text {h}\left(\text {v}-\text {v}_{\text {o}}\right)\)

This kinetic energy of the emitted electrons is proportional to the frequency of the light, contrary to the prediction of classical physics. 

Wave Nature of Electromagnetic Radiation

A wave is an energy-transmitting periodic oscillation that travels through space. Anyone who has been to a beach or thrown a stone into a puddle has seen waves moving through the water. These waves are created when energy is transferred to the water by wind, a stone, or another disturbance, such as a passing boat, causing the surface to oscillate up and down.

Properties of Waves

Waves have the following characteristic properties-

They are periodic; they repeat regularly in both space and time.

The distance between the midpoints of two peaks or two troughs is the wavelength \((\lambda)\). Wavelengths are described by a unit of meters.

A wave’s frequency is the number of oscillations that pass through a specific place in a given amount of time. The most common units are oscillations per second \((1 / \text {s}=\text {s}1)\), which is known as the hertz in the SI system \((\mathrm{Hz})\).

A wave’s amplitude, or vertical height, is equal to half of its peak-to-trough height. The energy of the wave increases with its amplitude. Two waves with the same amplitudes but different wavelengths can exist, and vice versa. The distance travelled by a wave per unit time is its speed \((\text {v})\), which is measured in meters per second \((\mathrm{m} / \mathrm{s})\). The speed of a wave is given by the product of its wavelength and frequency:

\(\operatorname{\text {Speed}}(\text {v})=\lambda(\text {wavelength}) \times \mu(\text {frequency})\)

In terms of wavelengths per unit time, the wave with the shortest wavelength has the most (i.e., the highest frequency). When two waves have the same frequency and speed, the one with a larger amplitude contains more energy.

Electromagnetic Radiation

Electromagnetic radiation is energy transmitted or radiated over space in the form of periodic oscillations of electric and magnetic fields. All kinds of electromagnetic radiation, including microwaves, visible light, and gamma rays, travel at the speed of light (c), which is approximately \(3.00 \times 10^{8} \mathrm{~m} / \mathrm{s}\) in a vacuum. This is a million times faster than the sound speed.

All forms of electromagnetic radiation consist of perpendicular oscillating electric and magnetic fields.

Because all types of electromagnetic radiation have the same speed \((\text {c})\), the only thing that distinguishes them is their wavelength and frequency. Radio waves have wavelengths of \(10^{1} \mathrm{~m}\), while gamma rays, generated by nuclear processes, have \(10^{-12} \mathrm{~m}\). By replacing \(\text {v}\) with \(\text {c} / \lambda\), we can show that the frequency of electromagnetic radiation is inversely proportional to its wavelength:

\(\text {E}-\text {E}_{0}=\text {hv}=\text {hc} / \lambda\)

Wavelength reduces as frequency rises, whereas c, being a constant, stays unchanged. In the same way, as frequency decreases, the wavelength increases.

(a) The visible spectrum of electromagnetic radiation is a narrow band of light with wavelengths between \(400\) and \(700\) nanometers. 

(b) When white light passes through a prism, it splits into multiple wavelengths, each of which corresponds to a different colour in the visible spectrum.

Human eyes detect distinct wavelengths (or frequencies) of radiation as light of various colours, ranging from red to violet, in decreasing wavelength order within the visible range. A prism can separate the constituents of white light, which is a blend of all visible light wavelengths.

Wavelength \((\text {m})\)	Type of Radiation
\(10^{-12}\)	gamma ray
\(10^{-10}\)	x-ray
\(10^{-9}\)	UV, visible
\(10^{-6}\)	infrared
\(10^{-3}\)	infrared
\(10^{-2}\)	microwave
\(10^{0}\)	radio

Unlike visible light, which is essentially harmless to our skin, UV radiation, which has wavelengths of less than \(400 \,\text {nm}\), has enough energy to cause severe skin damage in the form of sunburn. The atmosphere’s ozone layer protects us from the harmful effects of highly powerful UV radiation by absorbing sunlight with wavelengths shorter than \(350 \,\text {nm}\).

Summary

The particle nature of light posed scientists with a dilemma. On the one hand, it could satisfactorily explain the photoelectric effect, but on the other hand, it was inconsistent with the known wave behaviour of light, which might account for interference and diffraction phenomena. Accepting the idea that light contains both particle and wave-like features, i.e., light has dual behaviour, was the only way to overcome this dilemma. This concept was totally unknown to scientists, and it took them a long time to become convinced of its validity. This page explains the wave nature and particle nature of electromagnetic radiation.

FAQs

Q.1. What is the dual behaviour of electromagnetic radiation?
Ans: Besides acting like waves, it works like a stream of particles (called “photons”) that have no mass. The photons with the highest energy correspond to the shortest wavelength.

Q.2. What is Black Body Radiation?
Ans: The radiation emitted by heated objects, particularly a blackbody, is known as blackbody radiation. A blackbody is an object that absorbs all visible light, infrared light, ultraviolet light, and other forms of radiation that strike it.

Q.3. How are the wavelength and frequency of electromagnetic radiation related?
Ans: Wavelength and frequency are inversely proportional to each other: wavelength \(×\) frequency \(=\) constant, and is equal to the velocity \((\lambda \text {f}=\text {c})\). 

Q.4. What is the relationship and equation between the energy and wavelength of electromagnetic radiation?
Ans: The shorter the wavelengths and higher the frequency corresponds with greater energy. So the longer the wavelengths and lower the frequency results in lower energy. The energy equation is \(\text {E} = \text {hν}\).

Q.5. Which two factors of electromagnetic radiation have an inverse relationship?
Ans: Wavelength and frequency are inversely proportional. As the wavelength increases, the frequency decreases. 

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