Electromagnetic Radiations: Theory, Properties & Spectrum

‘Electromagnetic radiations can be defined as the radiations produced by the movement of particles that are electrically charged along with the matter or vacuum.

• The electrical fields and magnetic fields are at 90^o with each other.
• Earlier only two types of waves or radiations were considered: Electric Waves and Magnetic Waves.
• It was James Clerk Maxwell, a Scottish Physicist, who constituted the theory of Electromagnetic Radiation in 1864.
• Electromagnetic radiations originate from the electromagnetic field. 
• When electromagnetic radiation takes place, it is then released in form of photons.

These can be termed bundles of light energy or quantized harmonic waves that typically travel at the speed of light. On basis of the wavelength of the electromagnetic spectrum, the energy can be grouped into different categories. 

Key Terms: Electromagnetism, Electromagnetic Radiations, Electromagnetic Spectrum, Frequency, Radio Wave, Magnetic Waves, Speed of Light, Velocity, Frequency

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What is Electromagnetic Radiation?

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Electromagnetic Radiation can be defined as:

• The magnetic and the electric fields generally meet 90°.
• Thus, the combined waves start to move perpendicular to both electric and magnetic oscillating fields which take place during the disturbance.
• An electromagnetic wave can be characterized by the intensity it has, along with the frequency ν of the time variation of both electric and magnetic fields.
• As per Modern Quantum Theory, electromagnetic radiation is known as the flow of photons (also termed, light quanta) via space.
• Photons are packets of energy packets (hν) which move with the universal speed of light. 

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Theory of Electromagnetism

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The first theory of Electromagnetism was proposed by Clerk Maxwell, a Scottish physicist in 1864. However, it was experimentally confirmed by German Physicist Heinrich Hertz in 1887. In 1901 Marconi demonstrated long-range radio wave transmission between Newfoundland and Britain and in 1906 Reginald Fessenden produced the first modulated radio transmission with voice and music.

Thus, Maxwell constituted a theory showing a close relationship between Electric and magnetic waves and proved that these two waves play a significant role in and symmetric role in nature. He also hypothesized that a changing electric field would produce a magnetic field. He is the one who concluded that light and other electromagnetic waves consist of fluctuating electric and magnetic fields. He also calculated the speed of light to be 3 x 10⁸ m/s.

There were four main electromagnetic interactions discussed in the theory of Electromagnetism.

• The electric charges consisting of the force of attraction or repulsion between them are inversely proportional to the square of the distance between them.
• Magnetic poles are present in pairs that attract and repel each other, the same as electric charges.
• The direction of the magnetic field produced by an electric current depends on the direction of the electric current.
• A magnetic field is created by a moving electric field and vice versa.

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Electromagnetic Radiations Properties

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Below are the properties of electromagnetic radiation.

• Electromagnetic waves are transverse waves i.e., electric and magnetic fields are perpendicular to each other.
• It can travel without any medium or can travel through any medium including the vacuum.
• It travels at a constant speed of 2.99792458 x 10⁸ m/s. this velocity is also referred to as the speed of light.
• Electromagnetic waves and other forms of waves have common properties. The properties are refraction, reflection, interference, and Diffraction.
• It also act like particles in addition to waves.

Electromagnetic Wave

Wave Model

Electromagnetic radiation moves in the form of transverse waves in isotropic and homogenous media. A transverse wave indicates that the given direction of energy transfer is typically found to be perpendicular to the wave oscillations. Maxwell equations help to specify the magnetic production from the electric part and vice versa. Hence, the strengths of the fields are in a fixed ratio to meet Maxwell’s equations.

• In the case of far-field electromagnetic radiation, the time change in one field is known to be proportional to the space change in the other field.
• This demands both fields to be in phase.
• A single frequency’s electromagnetic wave is called a monochromatic wave which contains successive crests and troughs.
• Wavelength can be defined as the distance between two consecutive troughs or crests.
• Electromagnetic waves can vary in size and can have different wavelengths.

Thus, the wavelength of an electromagnetic wave is considered to be inversely proportional to the frequency:

Here,

• ν = frequency of the wave
• λ = wavelength of the wave
• f = frequency of the wave

Particle Model

The black body radiation has been seen to contradict the wave model of light. Max Planck, in the year 1900, proposed a theory of black-body radiation. It demonstrated the observed spectrum. The theory is on the basis of the emission of light occurring in discrete packets of energy. The energy of a photon is observed to be proportional to the frequency of the wave that represents it. Thus,

Here, 

• h = Planck’s constant
• f = frequency of the wave
• λ = wavelength
• E = energy of the photon

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Spectrum of Electromagnetic Radiation

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The range of wavelength and frequencies is known as the Electromagnetic Spectrum.

• The Electromagnetic spectrum is divided into seven parts in the order of reducing wavelength and increasing frequency and energy.
• The Electromagnetic spectrum includes Microwaves, Infrared waves, Radio waves, Ultraviolet waves, Visible Light, X-rays, and Gamma rays.
• The low-energy radiation waves are expressed in terms of frequency (Radio Waves).
• The high-energy radiation waves are expressed in terms of energy per photon (X-rays, Gamma rays). Infrared, UV light, Microwaves, and visible lights are expressed in terms of wavelength.

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Characteristics of Electromagnetic Radiation

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Electromagnetic radiation takes place when an atomic particle, for example an electron, has been accelerated by an electric field, thus making it accelerate. The nature of the electromagnetic wave is characterized by the following properties:

Wavelength

Wavelength is defined as the distance between the identical point or crest i.e., the distance between the top of one crest and to the top of the next crest is called wavelength. With a decrease in the wavelength, the frequency and energy increases. Wavekength is simply the distance of one full cycle of an oscillation.

Thus, c = \(\lambda\) v

• \(\lambda\) = wavelength
• c = speed of light
• v = frequency

Frequency

The number of times a wave passes through a point in a given unit of time is called frequency. Frequency is calculated in hertz or sec?¹. 

• E = hv
• E = energy
• h = Planck’s constant
• v = frequency that can be derived from the above relation

Velocity

Velocity is the product of wavelength and frequency and is always expressed as 3 x 108 m/s.

Velocity = \(\lambda\)v

Amplitude

Amplitude is defined as the distance between the highest point in a wave and the lowest point in the same wave. The larger is the amplitude, the higher the energy, lower the amplitude, and lower the energy is going to be. 

Time Period

The time taken by a wave to complete one wavelength is called the time period.

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Things to Remember

• Electromagnetic radiations can be defined as the radiations produced by the movement of particles that are electrically charged along with the matter or vacuum.
• Electromagnetic waves are transverse waves i.e., electric and magnetic fields are perpendicular to each other.
• Electromagnetic radiations can travel without any medium or can travel through any medium including the Vacuum.
• The velocity of electromagnetic radiation is 3x10⁸ m/s.
• The Electromagnetic spectrum is divided into seven regions in the order of decreasing wavelength and increasing frequency and energy.
• Wavelength, Frequency, Velocity, Amplitude, and Time Period characterize Electromagnetic radiation.

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Previous Year Questions

1. If a current of 2.0A flows through the smaller loop, then the flux linked with bigger loop is…… [JEE Main 2013]
2. A coil of cross-sectional area A having n turns is placed in a uniform magnetic field B….. [JEE Main 21018]
3. A copper rod of mass m slides under gravity on two smooth parallel rails, with separation ll ….. [JEE Main 2018]
4. Which is having minimum wavelength...[NEET 2002]
5. The speed of radio-waves is equal to….. [JIPMER 1998]
6. Gamma rays and visible light waves rays are a,ba,b and cc ….[UPSEE 2016]
7. A copper wire is wound on a wooden frame, whose shape …. [JEE Main 2019]
8. A metallic rod of length ll is tied to a string of length 2l and made to rotate with …. [JEE Main 2013]
9. A square frame of side 10 cm and a long straight wire carrying current 1 A are in the …. [JEE Main 2014]
10. If the rod makes n rotations per second, then the time averaged magnetic moment of the rod is… [JEE Main 2019]

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