Refraction: What Causes EM waves to bend?


Learning Objectives

At the end of this module, you should be able to:



This module goes more in depth into the physics of how refraction occurs. It is not necessary to do this module in order to understand the rest of the modules, but is included for those who want that in-depth understanding.

The bending of EM rays is called refraction. All EM radiation in the atmosphere refracts to some degree. Much of this course will concern the causes and effects of refraction on EM propagation. The following pages in this module will provide some background on the causes of refraction. The material on this page is theoretical in nature, but will be provided using animations and graphics, not mathematical equations. By understanding the various causes of EM refraction, you will be better able to apply the later course material to practical uses.

Refraction is caused by the response of charged particles to EM radiation. As we have seen, accelerating charges such as electrons generate EM waves. These EM waves in turn cause movement of other electrons at some distance from the generation source. When these latter electrons move, they absorb some of the original EM energy, but they also create new EM waves of their own. This process is sometimes called reradiation. It is the reradiation that causes the EM energy to be refracted. The reason for this is quite complex, but with the use of some diagrams, you should be able to gain a basic understanding of how the reradiation of initial EM causes refraction.


Bounded Electrons and Free Electrons

Electrons can exist in two forms: bounded or free. Bounded electrons are closely connected to individual atoms or molecules. This is because the nuclei of atoms contain positively-charged protons that attract the negatively-charged electrons. The electrons in nonmetals, including all the gases in Earth's troposphere, contain only bounded electrons. Learn more about bounded electron refraction here.

Free electrons are not closely connected to a particular atom or molecule; they move freely among various atomic nuclei. Metals contain large numbers of free electrons. This makes metals good electric conductors; electricity is the movement of free electrons. The ionosphere also contains free electrons. This is not because there are metals in the ionosphere. The reason is that the atoms and molecules in the ionosphere absorb large amounts of radiation (primarily UV) from the sun, which makes the electrons so energetic that they are not held to one nucleus. Matter which consists of free electrons is called plasma. Learn more about free electron refraction here.


Phase Speed and Index of Refraction

Earlier we discussed how EM radiation has wave characteristics. The phase speed of a wave is how fast the phase lines (i.e. wave crests or troughs of the EM field) move. In outer space, the phase speed is a constant, it is "the speed of light", often indicated by the letter "c". In the atmosphere, the phase speed of EM radiation varies. Although this variation is minuscule, it is still significant enough to cause important effects on EM propagation.

When EM radiation passes through a medium that causes variations in phase speed it tends to bend in the direction of the slower phase speed. You can visualize this by imagining that the waves in the faster part of the medium and the associated phase lines move faster compared to the same phase lines that are in the slower part. This bends the phase lines toward the slower waves. Since the EM energy travels at right angles to the phase lines, this means that the EM signal will bend (refract) toward the region of slower phase speeds.

So we see that refraction is related to the phase speed. We define the index of refraction as the speed of light in a vacuum divided by phase speed. So EM waves will tend to bend toward regions of higher index of refraction.


Study Questions

  1. Does oxygen in the troposphere contain many free electrons?
  2. Are there free electrons in an antenna?
  3. A part of the atmosphere has a lower index of refraction at lower elevations. Will EM radiation bend up or down?
  4. What is the index of refraction for outer space?

Continue to Module 2