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Energy comes in many forms, and we are familiar with the fact that energy can neither be created nor destroyed but can be converted into various forms. Any object that is in motion possesses kinetic energy and objects like cells and batteries have energy stored in them, which is known as potential energy. When charged particles like protons and electrons are at rest i.e., they are static, the electrostatic force comes into existence. When these particles are in motion, electromagnetic fields are produced, and these fields are responsible for transporting the kind of energy we call electromagnetic radiation.
Electromagnetic Radiation
An electric and magnetic disturbance that travels at a speed of about 3108m/sec is known as electromagnetic radiation. Electromagnetic radiation consists of energy packets known as quanta and the particles in these energy packets are known as photons. Photons have zero rest mass.
Some important features of electromagnetic waves are as follows:
The oscillation of electric and magnetic fields produces EM waves. Both the components of electric and magnetic fields are mutually perpendicular to each other and lie in the same plane.
While dealing with EM waves, we take their frequencies, wavelengths, or wave number into consideration.
E=h, known as Planck’s law, states that the energy of electromagnetic radiation is directly proportional to its frequency.
Electromagnetic radiation requires no medium to travel. It can travel through liquid, solid, and vacuum.
One of the properties of EM waves is the phenomenon of dispersion. The splitting of light, when it passes through a prism, into its constituent colors is known as dispersion.
Expressions Related to EM Radiation
Frequency: The number of waves that pass through some fixed point in one second is known as frequency. It is expressed in Hertz which is equivalent to cycles per second. Frequency can also be expressed in terms of the time period, i.e, f=1/T.
Wavelength: The distance between two successive crests and troughs is known as wavelength. The SI unit of wavelength is meter, but usually, in the case of EM radiation, we express wavelength in terms of angstroms or nanometers.
Wavenumber: The total number of waves that can pass through 1 m of linear space is termed as wave number. It’s the reciprocal of wavelength, and it is expressed in m-1.
Energy: Different radiations have different energies and it can be calculated using the above-mentioned law, i.e, Planck’s law. E=h, where, h is the Planck’s constant (=6.62610-34 Js-1).
Formulas for Electromagnetic Radiation
There exists a relationship between the various terms that have been mentioned above. Let’s write those formulas.
The symbol used for expressing wavelength is (Lambda). The relationship between frequency and wavelength is shown below.
λ=c/
Where c is the speed of light .
And, is the frequency of electromagnetic radiation.
The SI unit of wavelength is meter.
Linear frequency is represented either by f or (nu).
v=c/λ
The SI unit of frequency is per sec.
Wave number is represented by and the relationship between wavelength and wave number is
λ=1/λ
The SI unit of the wave number is per meter.
Using the formula for calculating the frequency in Planck’s law, we get
E=hc/λ
The SI unit of energy is Joules.
Wien’s Law states that the wavelength of maximum emission= 2900/ object’s temperature in Kelvins.
E=T4is the Stefan Boltzmann law where =5.67037441910-8 watt/m2K4.
We’ll be using these formulas in the problems at the end.
Electromagnetic Spectrum
Electromagnetic waves are categorized according to their frequency or according to their wavelength. Visible light has a wavelength range from ~400 nm to ~700 nm. Violet light has a wavelength of ~400 nm and a frequency of 7.5 1014 Hz. All these waves form an electromagnetic spectrum.
EM waves are classified based on their frequencies or wavelengths:
Radio waves: Radio waves have the longest wavelength of all the EM waves which makes them a perfect fit for radio and broadcasting stations. Radio and television stations and cell phone companies all produce radio waves that carry signals to be received by the antennae on the television, radio, or cellphone. Radio waves are also emitted by stars and gasses in space.
Microwaves: Microwaves have the second-lowest frequency in the EM spectrum. As their wavelength is somewhat less than that of radio waves, they range from a few centimeters up to a foot. Microwaves find their application in ovens, radar, electronic imaging for medical diagnosis, GPS, etc.
Infrared waves: These waves lie in the lower middle range of frequencies in the EM spectrum. The wavelengths of infrared waves range from a few millimeters down to microscopic lengths. The shorter-wavelength infrared waves are used in imaging technologies because they do not produce much heat whereas the longer-wavelength infrared waves are emitted by fire and the sun.
Visible light rays: Our eyes can only see in this range of the EM spectrum. The wavelength that is absorbed by an object determines its color.
Ultraviolet rays (UV rays): Ultraviolet radiation is emitted by the sun and is the reason for skin tans and sunburns. They have even shorter wavelengths than visible light rays. UV rays have proven helpful for scientists in learning about the structures of galaxies.
X-rays: X-rays are extremely high-energy waves with wavelengths between 0.03 and 3 nanometers. They have a very good penetrating power and can penetrate through almost any object. Natural sources of x-rays include enormously energetic cosmic phenomena such as pulsars, supernovae, and black holes. X-rays are commonly used in imaging technology to create pictures of the inside of our body; for example, to detect fractures in the bones.
Gamma rays: Gamma rays have the highest frequencies and the shortest wavelength in the EM spectrum. They possess extremely high energy because of their high frequencies. These waves are mostly emitted by the most energetic cosmic objects like a supernova, black holes, etc. Gamma rays are so powerful that they can easily destroy living cells. Doctors use gamma-ray imaging to look inside the human body.
Problems on Electromagnetic Radiation
The body has a temperature of 1000K. What is the body’s wavelength of maximum emission?
Solution: To solve this question, we’ll be using Wien’s law.
Wavelength of maximum emission= 2900 / object’s temperature in Kelvins
wavelength of maximum emission= 2900/1000
=2.9 microns
Find the energy that is emitted by an object that has a temperature of 50K as compared to an object having a temperature of 10K.
Solution: Using the Stefan Boltzmann law:
E=T4
E1=(504)=(625104) K 1
Also,
E2=(104)=(10000) K 2
Comparing 1 and 2,
E1/E2=(625104)/104
=625
Therefore, the energy emitted by an object at the temperature of 50K is 625 times more than that of the object at 10K.
Conclusion
Electromagnetic radiation is useful in many aspects. Using the relationships between frequency and wavelength or wavenumber and wavelength, we can easily calculate energies, frequencies and wavelengths of electromagnetic radiation. Different EM waves are classified based on their frequencies or wavelengths and different waves have different properties and uses. Electromagnetic waves can be split into a range of frequencies. This is known as the electromagnetic spectrum.
