PHYS 2212 Module 12.5

The Electromagnetic Spectrum

Recommended Reading

12.5 The Electromagnetic Spectrum

Learning Objectives

By the end of this section, you will be able to:

  • Explain how electromagnetic waves are divided into different ranges, depending on wavelength and corresponding frequency
  • Describe how electromagnetic waves in different categories are produced
  • Describe some of the many practical everyday applications of electromagnetic waves

The Electromagnetic Spectrum

You’ve all probably seen this picture, or one like it, before. Now let’s make sense of it.

This is the electromagnetic spectrum and it shows how we define types of EM waves based on their wavelength and frequency. Since the speed of an EM wave is

and since c is constant, the wavelength and frequency are inversely proportional to each other. An EM wave with a long wavelength has a small frequency and vice versa. The frequency of an EM wave will depend on the source of the wave. For example, the waves we produce using a dipole antenna (like in the first part of this module) are typically in the radio wave part of the EM spectrum. They have a frequency of somewhere around 104 Hz. This frequency is the same as the frequency of the AC generator we used with the antenna. The AC generator moves the electrons up and down in the antenna, which produces the E and B fields of the EM wave. The E and B fields change in magnitude and direction at the same rate that the AC generator moves the electrons up and down. So the frequency of the EM wave is equal to the frequency of the source that produced it. 

If I used a higher frequency AC generator with my dipole antenna, I could produce EM waves with higher frequency, like microwaves, which have a frequency around 108 or 109 Hz. That’s how the microwaves are generated in your microwave oven – with something like a dipole antenna that has a higher frequency than a radio antenna. It’s actually called a magnetron.

Basically all EM waves are produced by making electrons wiggle really fast. The faster they wiggle, the higher the frequency. Wiggle those electrons really fast and you can produce visible light, UV, and even x-rays and gamma rays. 

Notice on the EM spectrum that as the frequency increases (to the left), the wavelength decreases. And EM waves with higher frequency / shorter wavelength are also higher energy EM waves. That’s why x-rays and gamma rays have enough energy to pass through stuff while radio waves do not. And x-rays and gamma rays have enough energy to ionize molecules in human tissue – which can lead to sunburn or cancer depending on how much, while a radio wave will never have enough energy to give you a sunburn.

Pause & Predict 12.5.1
For x rays in vacuum, what is the wavelength, λ, if the frequency of the x rays is 9.44 × 1017 Hz?
Pause & Predict 12.5.2
What is the wavelength of this orange light when it travels through diamond (n = 2.42)?


Practice 12.5.1
An electromagnetic wave with an electric field that oscillates with a period of 8.43 ps is travelling in a vacuum. What is the wavelength of this electromagnetic wave?
Check your answer: B. 2.53 mm
Practice 12.5.2
An electromagnetic wave with a frequency of 4.72 x 1014 Hz has a speed of 1.71 x 108 m/s when it travels through a medium. What is the index of refraction of that medium?
Check your answer: D. 1.75
Practice 12.5.3
An electromagnetic wave with a frequency of 4.72 x 1014 Hz has a speed of 1.71 x 108 m/s when it travels through a medium. What is the wavelength of this electromagnetic wave when it is in the medium?
Check your answer: D. 362 nm

Absorption, Reflection, and Transmission of EM Waves

Materials can absorb, transmit, and reflect EM waves. So, how do you see an object? Why does it appear to be a specific color? Take this circle:

We observe this circle to be blue. The materials that make up this object (pigment, dye, etc) absorb light with frequencies corresponding to all the other frequencies and reflect the EM waves with frequencies corresponding to blue (~ 1015 Hz in the EM spectrum).

Actually, on a microscopic scale, the E-field in the EM wave makes the electrons in the material oscillate (just like the electrons in an antenna). The electrons in this material oscillate at a frequency of about 1015 Hz. Those oscillating electrons in the object act like the electrons in a dipole antenna and emit EM waves with f ~ 1015 Hz. When those 1015 Hz EM waves reach our eyes, we sense them as blue! This is what we have always just called reflection. The light doesn’t “bounce” off the object, but instead it excites the electrons and makes them wiggle with the same frequency and radiate the EM wave back at you.

We observe this circle as red. The materials in this object have electrons that oscillate at approximately 1014 Hz. When 1014 Hz EM waves hit this object, the electrons oscillate at that frequency and then those oscillating electrons emit EM waves with f = 1014 Hz. When those EM waves hit our eyes, we sense red.

Could Superman really use x-ray vision? How would it possibly work?

First, Superman would need to be able to “see” x-rays in the way that we “see” visible light. This might be possible since he is from Krypton and not human. His biology will be different and he might have x-ray sensors in his eyes. Second, the objects that he observes with his x-ray vision would have to emit x-rays (in a manner similar to how the blue circle emits blue light). This means there would have to be a source of x-rays to illuminate the object, in order for Superman to actually “see” the object with x-ray vision. Regular light bulbs don’t emit EM waves in the x-ray region, so they can’t be a source of x-rays. The Sun emits x-rays, but those are absorbed in our atmosphere. If Superman could emit x-rays, say from his eyes, he could illuminate objects with x-rays, and then the illuminated object would “reflect” x-rays back at him and he would “see” the object.