Astronomy For Beginners: Radiation

If you love Astronomy then looking at the sky is a regular task for you. You see all those stars and many other objects. What no one knew for a very long time is that we only see a small fraction of what the night sky is showing us. The reason why is that the light we see with our eyes is just one form of radiation. We now know of many other kinds. If you want to know more then read on and I will explain.



Radiation is the transmission of energy. At first we thought it through particles but now we know it is through waves. It can be between any 2 points or distance. The term electromagnetic is often applied to radiation and rightly so. This is because radiation energy is carried by electric and magnetic fields. There are many types of radiation as denoted by the electro-magnetic spectrum.






Light is a form of radiation. We also call this visible light. It is the light we can see with our own eyes. The kinds of radiation or light that we can not see take special instruments to detect and measure. Some examples are radio, infrared, ultraviolet, x-rays, and gamma rays.

  • Radio - these waves are the largest. We use them to carry radio and tv signals.
  • Microwave - these waves have a length between a millimeter and a meter.
  • Infrared - wavelengths between 12 centimeters and 740 nanometers.
  • Visible Light - these wavelengths are between 740-400 nanometers.
  • Ultraviolet - these occupy the region of 400-10 nanometers.
  • X-rays - these range from 10 to .01 nanometers.
  • Gamma rays - These are the smallest waves at less than a picometer.

Waves of Light

All forms of light travel in a wave motion. A wave is a disturbance of the medium in which it travels. That is the simplest definition I can think of. The medium does not move. It is the energy that moves through the medium. That is a wave.

All waves have certain characteristics that can be measured. These are the period, frequency, wavelength, and amplitude.

  • Period= time needed for the wave to repeat itself
  • Wavelength= distance needed for the wave to repeat itself
  • Amplitude= height of the wave
  • Frequency= The number of waves that pass any given point in a certain amount of time

The Parts of Visible Light

All light is a combination of different colors. White light is, in fact, made up of red, orange, yellow, blue, green, and violet. These different hues and their resulting frequencies is how we see light. Each of those colors has a different frequency with red having the lowest and violet the highest frequency. These colors are what we call the visible spectrum. Any light or radiation outside of these boundaries can not be seen by our eyes.

Electric Fields

The forces in an electric field can be either attract or repel each other. It depends on the charge of each individual component. Whether the particle is positive or negative it will put out an electrical force around it. If two particles encounter each other they can either attract or repel away from each other. If the charges of the two particles are the same then they will repel each other and if they are different then they will attract each other. This happens because the electric field of an object extends out in every direction at once. This means it extends out radially in every direction. The strength of this field also decreases with distance.


Electromagnetic Waves



James Maxwell was the first to figure out what these waves were. He developed a set of equations to deal with their effects. He combined the knowledge of electric fields and magnetic fields. You get a magnetic field every time an electric field changes. They change all the time so you actually have a magnetic field all the time too. So to expand on our wave model of light, these waves are formed of electric and magnetic fields traveling together. Interestingly enough, they travel perpendicular to each other. Together these two fields make up what we know as electromagnetism.

This is now an electromagnetic wave. These wave move at the speed of light which is roughly 300,000 km/s. That is very fast but it is not instant. Of course you could only tell that at long distances. Just something for you to ponder though.

Properties of Light

Light is made up of photons. These exhibit both particle and wave characteristics.

Electromagnetic Spectrum

As you now know there are several kinds of electromagnetic radiation. Some of it is visible but most of it is not. They have their own characteristics too, meaning we can tell them apart with certain tests. These differences are wavelength and frequency. So we have to measure these things with instruments and tests. Radio frequencies are on the low end of the spectrum and gamma rays are on the high end.

Spectrum of Radiation

This is all energy that moves as waves. Remember that each wave consists of electric and magnetic fields. These fields move together at the speed of light which is 300,00 km/s. It is collectively called an electromagnetic wave. The wavelengths of radiation vary greatly by the way. Radio waves are quite long while gamma rays are atomic in scale. Another key point to know is that objects emit radiation in a variety of wavelengths. Some of it might be in visible light but most will be some other form of electromagnetic radiation.

Opacity of Objects

The opacity of an object is how little light or radiation it lets through. This concept directly affects what we are able to perceive on this planet. How this affects us is that even in the visible spectrum there will be more opacity and lower amounts of radiation that is let through. Then in other parts the opposite will be true and it will be less opaque.

Thermal Radiation

Thermal radiation is energy given off by heat. Every object gives off at least a little bit of heat too. This happens because electromagnetic radiation is given off at the particle level. This is also directly related to the object's temperature. The hotter the temperature the more electromagnetic radiation is given off. Every object has a heat signature then. This is how you can detect what kind of object you are seeing when you look at its heat analysis.


Blackbody Analysis




No physical object emits all of its radiation at one frequency; it is actually scattered around a lot. This is the heat signature I referred to above. Every object will look a little bit different. While signatures do have a lot in common usually, there are always differences. Just don't expect them to be radically different because they are not. When graphed this is called a blackbody curve.

Characteristics of Radiation

When graphing blackbody curves of objects you will notice that hotter items shift the curve upwards. Stars are a great example of this. Hotter temperatures are higher frequencies too. This is why we see objects when they are very hot. Objects with less temperature or cooler emit radiation in the non-visible spectrum. It is all still radiation though. There is also a simple relationship between the temperature of the object and the wavelength the radiation is admitted at.


 $$ wavelength=\frac {1}{temperature} $$

As you might infer from this, the hotter the temperature the greater the amount of energy is invovled. This is summarized by the equation:

$$ energy = temperature^4 $$

Uses In Space

Scientists use the blackbody curves to determine the temperature off far off objects in space. Since we can not observe it directly that is the best we can do. We can, however, infer a lot of information from an object's temperature. Comparing how two objects are different also helps us classify too.


Electromagnetic radiation is the basis for a lot of interesting science. Understanding a little about it will tell us a lot about objects in space where we can not go. If you enjoyed this article on radiation then please share it. Also here are the posts that came before it if you want to read them in order.