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Every time you see those annoying YouTube ads about the best ways to waste your time or every time you have to see your grade cards, have you ever asked yourself – why in the world am I seeing this? I know I have. After years of research, I have reached the conclusion that we see what we do and don’t want to see solely because of a little something called electromagnetic waves, more specifically, the visible portion of electromagnetic waves. This is where something called as the electromagnetic spectrum comes into existence.

You know ALL about electric and magnetic fields, don’t you? Yeah, neither do I, but please read on. So it would seem that every time you have an oscillating electric field, there’s an oscillating magnetic field which is generated perpendicular to said electric field. Stay with me. This oscillating magnetic field, in turn, generates an electric field perpendicular to it and so on. So this effectively married couple is what is known as an electromagnetic wave.

Electric wave + Magnetic wave = Electromagnetic wave (Take your time :P)

All this mayhem about electric and magnetic fields generating electromagnetic waves is making my head mushy. Don’t you think we need to know a little bit more about wave properties before moving on to electromagnetic spectrum? Let’s do that first.

Basic Properties of Waves: Amplitude, Frequency, and Wavelength

As you might already know (if not, chill :P), a wave has a trough (the lowest point of the wave) and a crest (the highest point of the wave). The wavelength of the wave is the horizontal distance between two consecutive troughs or crests. The amplitude of the wave is the vertical distance between a trough or a crest and the wave’s central axis or the reference line. This is the property which is responsible for varying brightness of the light. The frequency of a wave refers to the number of full wavelengths that pass by a given point in space every second. It is the inverse of time. It has the units Hertz, which is equivalent to 1/seconds (obviously!).

For electromagnetic radiation, the relation between the wavelength and frequency is c=λ×ν, where λ is the wavelength, ν is the frequency of the electromagnetic radiation and c is a constant which is the speed of light. This relationship is a result of the important fact that all electromagnetic radiation, regardless of wavelength or frequency, travel at the speed of light. For all other waves, λ×ν equals the speed of the wave.

The frequency of waves can also be reflected in another property of waves, the period. A wave’s period is the length of time it takes for one wavelength to pass by a given point in space. Mathematically, the period () is simply the reciprocal of the wave’s frequency :

T=1/ν
The SI unit of period is seconds(s).

An interesting fact about frequency is that it is directly proportional to energy:

E=h×ν

where ‘h’ is called the Planck’s constant. It is a consequence of quantization of electromagnetic radiation. I’m sure many of you must be dumbfounded by all these new terms. Stay with me, I’ll explain what quantization of electromagnetic radiation is.

Quantization of Light

So far we dealt with properties of light when it behaves as a wave. An astonishing discovery was made by Max Planck who showed that light exhibited particle nature too. (Take your time digesting that one :P). By observing radiation of black bodies (bodies which completely absorb all light that falls on them) he concluded that the energy of electromagnetic radiation is not continuous as we would expect from its wave nature, rather it is quantized, or in other words, in multiples of a number. He concluded that the energy is a function of frequency, as we saw earlier and gave the formula: E=h×ν. Thus the concept of photon was developed. Photons are massless particles which are the elementary quanta of light. Thus light exhibits a dual nature – particle and wave.

Now enough about the properties of waves and quantization principle. Let’s move on to the electromagnetic spectrum.

Electromagnetic waves are way more important than you’d think. Another name for electromagnetic waves is light. Light does more than help us watch those ground breaking YouTube ads, as visible light is a very small chunk of an effectively infinite spectrum. Different types of electromagnetic waves are classified based on what frequency range they fall into, and the frequency of the wave can have dramatic influences on its properties, such as whether or not it can turn you into an Avenger.

 

The major types of waves are as follows:

important-questions-for-class-12-physics-cbse-electromagnetic-waves-6

As you can observe from the above picture, there are no precise accepted boundaries between any of these contiguous portions, so the ranges tend to overlap. Let’s look at what some of them are used for:

Radio waves: Used for broadcasting and long range satellite communication

Microwaves: Cooking, communication, satellite communication

Infrared: thermal imaging, short range communications, optical fibres, television remote controls, security systems

Visible Light: (Really?)

Ultraviolet: security marking, fluorescent lamps, detecting forged bank notes, disinfecting water

X-rays: observing the internal structure of objects, airport security scanners, medical X-rays

Gamma rays: sterilising food and medical equipment, detection of cancer and its treatment, becoming an Avenger or building Death Ray depending on fictional universe

The electromagnetic spectrum

Electromagnetic waves can be classified and arranged according to their various wavelengths/frequencies; this classification is known as the electromagnetic spectrum. The following table shows us this spectrum, which consists of all the types of electromagnetic radiation that exist in our universe.

The electromagnetic spectrum is comprised of all the varieties of radiation in the universe. Gamma rays have the highest frequency, whereas radio waves have the lowest. Visible light is approximately in the middle of the spectrum, and comprises a very small fraction of the overall spectrum.
The electromagnetic spectrum. Image from UC Davis ChemWiki, CC-BY-NC-SA 3.0
As we can see, the visible spectrum—that is, light that we can see with our eyes—makes up only a small fraction of the different types of radiation that exist. To the right of the visible spectrum, we find the types of energy that are lower in frequency (and thus longer in wavelength) than visible light. These types of energy include infrared (IR) rays (heat waves given off by thermal bodies), microwaves, and radio waves. These types of radiation surround us constantly, and are not harmful, because their frequencies are so low. As we will see in the section, “the photon,” lower frequency waves are lower in energy, and thus are not dangerous to our health.
To the left of the visible spectrum, we have ultraviolet (UV) rays, X-rays, and gamma rays. These types of radiation are harmful to living organisms, due to their extremely high frequencies (and thus, high energies). It is for this reason that we wear suntan lotion at the beach (to block the UV rays from the sun) and why an X-ray technician will place a lead shield over us, in order to prevent the X-rays from penetrating anything other than the area of our body being imaged. Gamma rays, being the highest in frequency and energy, are the most damaging. Luckily though, our atmosphere absorbs gamma rays from outer space, thereby protecting us from harm.
Next, we will talk about the relationship between a wave’s frequency and its energy.

Quantization of energy and the dual nature of light

We have already described how light travels through space as a wave. This has been well-known for quite some time; in fact, the Dutch physicist Christiaan Huygens first described the wave nature of light as far back as the late seventeenth century. For about  years after Huygens, physicists assumed that light waves and matter were quite distinct from one another. According to classical physics, matter was composed of particles that had mass, and whose position in space could be known; light waves, on the other hand, were considered to have zero mass, and their position in space could not be determined. Because they were considered to be in different categories, scientists did not have a good understanding of how light and matter interacted. This all changed in , however, when the physicist Max Planck began studying blackbodies – bodies heated until they began to glow.

Molten lava acts as a blackbody, emitting electromagnetic radiation in the visible region at very high temperatures.
Molten lava emitting blackbody radiation. Image courtesy of the U.S. Geological Survey.
Planck found that the electromagnetic radiation emitted by blackbodies could not be explained by classical physics, which postulated that matter could absorb or emit any quantity of electromagnetic radiation. Planck observed that matter actually absorbed or emitted energy only in whole-number multiples of the value h\nu, where h is Planck’s constant, 6, point, 626, times, 10, start superscript, minus, 34, end superscript, space, J, dot, s, and \nu is the frequency of the light absorbed or emitted. This was a shocking discovery, because it challenged the idea that energy was continuous, and could be transferred in any amount. The reality, which Planck discovered, is that energy is not continuous but quantized—meaning that it can only be transferred in individual “packets” (or particles) of the size h\nu. Each of these energy packets is known as a quantum (plural: quanta).
While this might sound confusing, we are actually already very familiar with quantized systems. The money we use daily, for example, is quantized. For instance, when you go into a store, you will not see anything on sale for a price of one dollar and two and a half cents left parenthesis, dollar sign, 1, point, 025, right parenthesis. This is because the smallest possible monetary unit is the penny—it is impossible to transfer money in any smaller amount than this. Just as we cannot pay the cashier at the store half of a cent, energy cannot be transferred in anything less than a single quantum. We can think of quanta as being like “pennies” of electromagnetic energy—the smallest possible units by which such energy can be transferred.
Planck’s discovery that electromagnetic radiation is quantized forever changed the idea that light behaves purely as a wave. In actuality, light seemed to have both wavelike and particle-like properties.

The photon

Planck’s discoveries paved the way for the discovery of the photon. A photon is the elementary particle, or quantum, of light. As we will soon see, photons can be absorbed or emitted by atoms and molecules. When a photon is absorbed, its energy is transferred to that atom or molecule. Because energy is quantized, the photon’s entire energy is transferred (remember that we cannot transfer fractions of quanta, which are the smallest possible individual “energy packets”). The reverse of this process is also true. When an atom or molecule loses energy, it emits a photon that carries an energy exactly equal to the loss in energy of the atom or molecule. This change in energy is directly proportional to the frequency of photon emitted or absorbed. This relationship is given by Planck’s famous equation:
E=h\nu
where E is the energy of the photon absorbed or emitted (given in Joules, J), \nuis frequency of the photon (given in Hertz, H, z), and h is Planck’s constant, 6, point, 626, times, 10, start superscript, minus, 34, end superscript, space, J, dot, s.

So that’s all about the electromagnetic spectrum in a nutshell. Everything you do has some reason to thank electromagnetic waves, from the latest Snapchat filter to your admiration Donald Trump’s wardrobe (Yes, that was a joke. Deal with it).

Conclusion

Electromagnetic radiation can be described by its amplitude (brightness), wavelength, frequency, and period. By the equation E=h\nu, we have seen how the frequency of a light wave is proportional to its energy. At the beginning of the twentieth century, the discovery that energy is quantized led to the revelation that light is not only a wave, but can also be described as a collection of particles known as photons. Photons carry discrete amounts of energy called quanta. This energy can be transferred to atoms and molecules when photons are absorbed. Atoms and molecules can also lose energy by emitting photons.
X-ray is one member of the huge wave family that the electromagnetic spectrum is.You can also read about the transverse waves. We hope this article about Electromagnetic spectrum helped you.

For more such articles keep reading Toppr Bytes. 

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