What are Electromagnetic Waves?
Electromagnetic waves are a form of radiation that travel though the universe. They are formed when an electric field couples with a magnetic field.
Both electricity and magnetism can be static (respectively, what holds a balloon to the wall or a refrigerator magnet to metal), but when they change or move together, they make waves. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.
Unlike sound waves, which must travel through matter by bumping molecules into each other like dominoes (and thus can not travel through a vacuum like space), electromagnetic waves do not need molecules to travel. They can travel through air, solid objects, and even space, making them very useful for a lot of technologies.
When you listen to the radio, connect to a wireless network, or cook dinner in a microwave oven, you are using electromagnetic waves. Radio waves and microwaves are two types of electromagnetic waves. They only differ from each other in wavelength – the distance between one wave crest to the next.
While most of this energy is invisible to us, we can see the range of wavelengths that we call light. This visible part of the electromagnetic spectrum consists of the colors that we see in a rainbow – red, orange, yellow, green, blue, indigo, and violet. Each of these colors also corresponds to a different measurable wavelength of light.
Waves in the electromagnetic spectrum vary in size from very long radio waves that are the length of buildings to very short gamma-rays that are smaller than the nucleus of an atom.
Their size is related to their energy. The smaller the wavelength, the higher the energy. For example, a brick wall blocks the relatively larger and lower-energy wavelengths of visible light but not the smaller, more energetic x-rays. A denser material such as lead, however, can block x-rays.
While it’s commonly said that waves are “blocked” by certain materials, the correct understanding is that wavelengths of energy are absorbed by the material. This understanding is critical to interpreting data from weather satellites because the atmosphere also absorbs some wavelengths while allowing others to pass through.
When Was Electromagnetism Discovered?
People have known about electricity and magnetism since ancient times, but the concepts were not well understood until the 19th century, according to a history from physicist Gary Bedrosian of the Rensselaer Polytechnic Institute in Troy, New York.
In 1873, Scottish physicist James Clerk Maxwell showed that the two phenomena were connected and developed a unified theory of electromagnetism. The study of electromagnetism deals with how electrically charged particles interact with each other and with magnetic fields.
Maxwell developed a set of formulas, called Maxwell’s equations, to describe the different interactions of electricity and magnetism. Though there were initially 20 equations, Maxwell later simplified them to just four basic ones. In simple terms, these four equations state the following:
- The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them.
- Magnetic poles come in pairs that attract and repel each other, much as electric charges do.
- An electric current in a wire produces a magnetic field whose direction depends on the direction of the current.
- A moving electric field produces a magnetic field, and vice versa.
Basic properties of waves: Amplitude, wavelength, and frequency
As you might already know, a wave has a trough (lowest point) and a crest (highest point). The vertical distance between the tip of a crest and the wave’s central axis is known as its amplitude. This is the property associated with the brightness, or intensity, of the wave.
The horizontal distance between two consecutive troughs or crests is known as the wavelength of the wave. These lengths can be visualized as follows:
Keep in mind that some waves (including electromagnetic waves) also oscillate in space, and therefore they are oscillating at a given position as time passes. The quantity known as the wave’s frequency refers to the number of full wavelengths that pass by a given point in space every second; the SI unit for frequency is Hertz (Hz), which is equivalent to “per seconds” (written as 1/s or s-1).
As you might imagine, wavelength and frequency are inversely proportional: that is, the shorter the wavelength, the higher the frequency, and vice versa. This relationship is given by the following equation:
c = λv
where λ (the Greek lambda) is the wavelength (in meters, m ) and v (the Greek nu) is the frequency (in Hertz, Hz ). Their product is the constant c , the speed of light, which is equal to 3.00 x 108 m/s . This relationship reflects an important fact: all electromagnetic radiation, regardless of wavelength or frequency, travels at the speed of light.
How Are Electromagnetic Waves Formed?
Generally, an electric field is produced by a charged particle. A force is exerted by this electric field on other charged particles. Positive charges accelerate in the direction of the field and negative charges accelerate in a direction opposite to the direction of the field.
The Magnetic field is produced by a moving charged particle. A force is exerted by this magnetic field on other moving particles. The force on these charges is always perpendicular to the direction of their velocity and therefore only changes the direction of the velocity, not the speed.
So, the electromagnetic field is produced by an accelerating charged particle. Electromagnetic waves are nothing but electric and magnetic fields travelling through free space with the speed of light c.
An accelerating charged particle is when the charged particle oscillates about an equilibrium position. If the frequency of oscillation of the charged particle is f, then it produces an electromagnetic wave with frequency f.
The wavelength λ of this wave is given by λ = c/f. Electromagnetic waves transfer energy through space.
Characteristics of Electromagnetic Waves
- Motion is similar to transverse waves, except that it does not require any medium to travel
- Consist of electric field and magnetic field perpendicular to each other and the direction of propagation
- Defined by wavelength, velocity, frequency, amplitude, and time-period.
- Velocity of EM waves in vacuum is a constant, c = 3 X 108 m/s
- While going from one medium to another, EM waves change their wavelength and hence, their speed
- Depending upon the type of radiation, EM waves have different sources. For example, x-rays are generated when high energy electrons impact a metal target.
Graphical Representation of Electromagnetic Waves
Electromagnetic waves are shown by a sinusoidal graph. It consists of time-varying electric and magnetic fields which are perpendicular to each other and are also perpendicular to the direction of propagation of waves.
Electromagnetic waves are transverse in nature. The highest point of the wave is known as the crest while the lowest point is known as a trough. In vacuum, the waves travel at a constant velocity of 3 x 108 m.s-1.
Mathematical Representation of Electromagnetic Wave
A plane Electromagnetic wave travelling in the x-direction is of the form
𝐸(𝑥,𝑡)=𝐸𝑚𝑎𝑥cos(𝑘𝑥−𝜔𝑡+Φ)
𝐵(𝑥,𝑡)=𝐵𝑚𝑎𝑥cos(𝑘𝑥−𝜔𝑡+Φ)
In the electromagnetic wave, E is the electric field vector and B is the magnetic field vector.
Maxwell gave the basic idea of Electromagnetic radiations, while Hertz experimentally confirmed the existence of an electromagnetic wave.
The direction of propagation of the electromagnetic wave is given by the vector cross product of the electric field and magnetic field. It is given as: E X B
Electromagnetic Wave Equation
- The electromagnetic wave equation describes the propagation of electromagnetic waves in a vacuum or through a medium.
- The electromagnetic wave equation is a second-order partial differential equation.
- It is a 3D form of the wave equation.
- The homogeneous form of the equation is written as
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.
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.
Types of Electromagnetic Waves
There are seven types of electromagnetic waves – radio wave, microwave, infrared, visible light, ultraviolet, x-rays, and gamma rays.
#1. Radio waves.
Radio waves are at the lowest range of the electromagnetic spectrum, with frequencies of up to about 30 billion hertz, or 30 gigahertz (GHz), and wavelengths greater than about 0.4 inch (10 millimeters). Radio is used primarily for communications, including voice, data and entertainment media.
#2. Microwaves.
Microwaves fall in the range of the electromagnetic spectrum between radio and IR. They have frequencies from about 3 GHz to 30 trillion hertz, or 30 terahertz (THz), and wavelengths of about 0.004 to 0.4 inch (0.1 to 10 mm).
Microwaves are used for high-bandwidth communications and radar, as well as for a heat source for microwave ovens and industrial applications.
#3. Infrared.
Infrared is in the range of the electromagnetic spectrum between microwaves and visible light. IR has frequencies from about 30 to 400 THz and wavelengths of about 0.00003 to 0.004 inch (740 nanometers to 100 micrometers). IR light is invisible to human eyes, but we can feel it as heat if the intensity is sufficient.
#4. Visible light.
Visible light is found in the middle of the electromagnetic spectrum, between IR and UV. It has frequencies of about 400 to 800 THz and wavelengths of about 0.000015 to 0.00003 inch (380 to 740 nanometers). More generally, visible light is defined as the wavelengths that are visible to most human eyes.
#5. Ultraviolet.
Ultraviolet light is the range of the electromagnetic spectrum between visible light and X-rays. It has frequencies of about 8 × 1014 to 3 x 1016 Hz and wavelengths of about 0.0000004 to 0.000015 inch (10 to 380 nanometers).
UV light is a component of sunlight, but it is invisible to the human eye. It has numerous medical and industrial applications, but it can damage living tissue.
#6. X-rays.
X-rays are roughly classified into two types: soft X-rays and hard X-rays. Soft X-rays make up the range of the electromagnetic spectrum between UV and gamma-rays. Soft X-rays have frequencies of about 3 × 1016 to 1018 Hz and wavelengths of about 4 × 10−7 to 4 × 10−8 inch (100 picometers to 10 nanometers).
Hard X-rays occupy the same region of the electromagnetic spectrum as gamma-rays. The only difference between them is their source: X-rays are produced by accelerating electrons, while gamma-rays are produced by atomic nuclei.
#7. Gamma-rays.
Gamma-rays are in the range of the spectrum above soft X-rays. Gamma-rays have frequencies greater than about 1018 Hz and wavelengths of less than 4 × 10−9 inch (100 picometers).
Gamma radiation causes damage to living tissue, which makes it useful for killing cancer cells when applied in carefully measured doses to small regions. Uncontrolled exposure, though, is extremely dangerous to humans.
Applications of Electromagnetic Waves
Following are a few applications of electromagnetic waves:
- Electromagnetic radiations can transmit energy in a vacuum or using no medium at all.
- Electromagnetic waves play an important role in communication technology.
- Electromagnetic waves are used in RADARS.
- UV rays are used to detect forged bank notes. Real banknotes don’t turn fluorescent under UV light.
- Infrared radiation is used for night vision and is used in security cameras.