Electromagnetic Spectrum

Electromagnetic Spectrum Definition

Let's start our journey by looking at the definition of electromagnetic spectrum.

In other words, the electromagnetic spectrum is a continuum of electromagnetic radiation, containing different wavelengths and frequencies. Each region of the electromagnetic spectrum consists of a different type of electromagnetic radiation.

For example, ultraviolet radiation has a different effect than microwave radiation, and, of course, both of these forms of radiation are different from the waves that we interpret as visible light!

Electromagnetic radiation is present all around us! Most EM radiations are actually invisible to the naked eye. Some examples of EM radiation include rays coming from the sun, and also X-rays.

Don't worry if nothing makes sense right now. We will learn about the electromagnetic spectrum in baby steps!

Wavelengths in the Electromagnetic Spectrum

We can describe waves in terms of wavelength and frequency. First, we need to talk about wavelengths in the EM spectrum.

In a wave, a crest is the highest point, or maximum value on the wave, whereas a trough is the lowest point or minimum value on the wave. The figure below shows a simple wave and its wavelength.

Fig. 1: A simple wave showing crests and troughs, Isadora Santos - StudySmarter Originals.

The wavelength is typically measured in units of distance. For example:

• Gamma rays, which have a very short wavelength, are measured in nanometers (nm).
• Radio waves, which have a longer wavelength, can be measured in centimeters (cm) or kilometers (km) depending on how far you go on the spectrum of radio waves.

Look at the figure below.

Fig. 2: As wavelength increases, the number of wave cycles per unit time decreases, causing frequency to decrease, StudySmarter Originals.

This image shows two different waves and the wavelengths between two adjacent crests. Notice that the top wave has a long wavelength compared to the bottom wave, which has a short wavelength.

Wavelength and frequency are said to be inversely proportional. This means that if wavelength increases, frequency decreases, and vice versa!

Frequency in the Electromagnetic Spectrum

Now, let's define frequency. Frequency is typically measured in units of hertz (Hz), which is equal to one per second (1/s or s^-1).

Let's go back to the image above, showing the two waves and their wavelengths. What can you tell about their wave cycles?

We can say that there are more wave cycles in the bottom wave and fewer wave cycles in the top wave. This means that the top wave has a lower frequency, and the bottom wave has a higher frequency.

• As wavelength increases, the number of wave cycles per unit time decreases, causing frequency to decrease.

Relationship Between Wavelength and Frequency

We learned that a wave with a shorter wavelength will have a higher frequency. This is because the wave cycle will be able to repeat more times per second! At a fixed speed, the frequency of a light wave is inversely proportional to its wavelength. This inverse relationship can be seen by the equation:

$$\lambda =\frac{v}{f}$$

Where:

• λ is the wavelength (in meters).
• f is the frequency (in cycles per second).
• v is the phase speed. The phase speed, in the case of electromagnetic radiation, is the speed of light. In a vacuum, this would be 2.9979 x 10⁸ m/s.

Let's look at an example!

Relationship between Energy, Wavelength, and Frequency

Now, let's explore the relationship between energy, wavelength and frequency, also known as Planck's relation.

$$E=h\cdot v$$

Where,

• E is the energy
• h is the planck's constant (6.63x10^-34 J·s)
• v is the frequency.

Through Planck's equation, we can relate the energy of a photon to the frequency of the electromagnetic wave. We write this relationship as:

$$E_{photon}=hv=\frac{hc}{v}$$

where E_photon is the energy of a photon, h is Planck's constant, v is the frequency, and c is the speed of light (3.0x10⁸ m/s). So, the general rule is that, when wavelength increases, both E and v decreases. You can learn more in-depth about this by checking out "Photoelectric Effect".

Electromagnetic Spectrum Diagram

Let's take a look at the electromagnetic spectrum diagram. In this diagram, we can see the spectrum range from gamma rays to radio waves, ranging from 0.0001 nm to 100 m. Additionally, we can see a wave with varying wavelengths and frequencies. Notice that as the wavelength increases, the frequency, and energy of the wave decreases.

• Gamma rays have the most energy, the highest frequency, and therefore, the smallest wavelength.

The visible light portion of the spectrum is shown from about 400 nm to 750 nm, with colors corresponding to their respective wavelengths. This is the region of the electromagnetic spectrum that we see as light!

Electromagnetic Spectrum: Examples

There are seven types of electromagnetic waves, and each of them has different properties. Note that there are no hard barriers between the regions of the EM spectrum. The different types of waves are separated into different regions based on their wavelengths, which gradually change as you move across the spectrum.

Let's look at some everyday chemistry examples involving electromagnetic waves.

Radio waves have the longest wavelengths and lowest frequencies of the different regions of the electromagnetic spectrum. They are commonly used in communication. For example, higher frequencies of radio waves are used for radar and television, while lower frequencies of radio waves are used for listening to the radio. Radio waves are also used in techniques such as MRI. Magnetic resonance imaging (MRI) combines radio waves and a magnetic field to create a picture of a cross-section of the body.

Similar to radio waves, microwaves are also commonly used in communication and radar, in addition to being used as a heat source.

Microwaves are also used in electron spin resonance for the induction and detection of electron paramagnetic resonance.

Infrared radiation also transmits heat. For example, infrared radiation is used to transmit heat from radiators, and in cooking to boil food. Infrared radiation can also be used to cause the bending and stretching of covalent bonds, and IR spectroscopy can be used to identify the presence of these different types of covalent bonds.

Visible light is the only portion of the EM spectrum that we see that causes different colors. The electromagnetic radiation given off by a wave with a wavelength of around 400 nm is interpreted as light by humans as the color purple, while electromagnetic radiation given off by a wave with a wavelength of around 700 nm is interpreted as light by humans as the color red.

Ultraviolet (UV) radiation is a huge part of the radiation emitted by the sun. A large amount of this radiation is absorbed by gases in the atmosphere; however, this radiation can also be absorbed by the skin, causing sunburn, skin damage, and cancer. UV radiation can be used to cause electron transitions within atoms and promote them to higher energy levels. This allows scientists to collect data about an atom's electronic structure and its electron shell model.

X-rays, like ultraviolet radiation, can be damaging to humans, but are also very useful in science. This form of radiation is used in medical machinery to see inside living and non-living things through the common technique of X-ray imaging.

Gamma rays have the shortest wavelengths and highest frequencies, and therefore, the highest energies. This region of the electromagnetic spectrum is also used in medicine as ‘radiation’ to attack cancer cells.

Now, I hope that you feel more confident in your understanding of the Electromagnetic Spectrum!