Radiation: "Space calling!"

A trickle of "photons" is all that an astronomer has to work with. A photon - a precious "package of energy" - may have traveled for millions of years to reach the eager astronomer. What secrets does it hold? In this first section you will be introduced to the underlying ideas of light and the electromagnetic spectrum.

Radiation

The term "radiation" has a variety of meanings, some of which may be alarming. You know that uranium or radium for example "emit radiation" which may be harmful. But what does this mean and what is being emitted? Fundamentally radiation is anything that "radiates" from a source. This means the term could be applied to "particles" or "waves". Table 5.1 summarizes this and provides some examples.

Particle
A piece of radium metal can emit alpha particles (helium nuclei) as well as beta particles (electrons). When the term "radiation" is used in the context of radioactivity it often means this. Radium (and most radioactive substances) also emits gamma radiation which is a high energy photon.
Wave

Sound or a vibration in a fluid can create waves which radiate outward. The following video clip simulates what a vibrating point in water would produce:

The disturbance radiates outward as a series of wave crests followed by wave troughs.

Light

or

Electromagnetic wave

or

Photon

A light bulb radiates light energy in all directions.

Light energy is often referred to as a photon. This is an elusive idea since it is neither but has properties of both wave and particle.

Table 5.1 Different kinds of radiation

Light - Wave or Particle?

The nature of light gets at one of the deepest questions in physics. Is light a wave or is it a particle? The answer is it is neither! Depending on how you interact with light it can display either wave or particle properties. Regardless of how you view light, however, remember that light is a form in which energy can be moved through space.

Light as a wave

When we look at light averaged over "long" periods of time (long could be as brief as a microsecond!) then we tend to see wave behaviours. Any wave has two very important defining numbers:

  • Wavelength is the distance between any two successive crests (high points) or troughs (low points) on a wave (see figure 5.1). The colour we perceive is directly related to wavelength. Blue light has a shorter wavelength than red light. The Greek symbol l ("lambda") is commonly used to denote wavelength.
  • Frequency is the number of waves created per second. Frequency is measured in units of Hertz (Hz).
Figure 5.1 Comparison of light waves of different colours

Frequency and wavelength are related to other by the speed of the wave. This can be expressed as , where v is the wave speed, f is the frequency and l ("lambda") is wavelength. This equation implies the following key connection between frequency and wavelength:

large or high frequency implies smaller wavelength
low frequency implies longer or larger wavelength

Example 5.1 Visible light travels with a speed of 3 x 108 m/s and consists of wavelengths between 400 nm and 700 nm. What are the corresponding frequencies for visible light?

Solution: Use the basic equation to find the link between wavelength and frequency. Re-arrange the equation and apply it to 400 nm wavelengths to read:

You can use the applet given in figure 5.3 to confirm this. Visible light has a freqeuncy in the range 4.3 x 1014 Hz to 7.5 x 1014 Hz.

Light as a particle

When light interacts with matter at an atomic level the particle behaviour of light becomes more important. The original idea of light as a "bundle" or quantum of energy is due to Albert Einstein. Eventually the term photon was applied to the idea of a quantum of light energy.

We often use the term "photon" to refer to light with the understanding that light can is an entity that has both wave and particle characteristics. Regardless of which set of properties we see exhibited it is the case that light is a form of energy and we can quantify the relation between light energy and the frequency of the light or the wavelength by the following formulae:

Energy versus Wavelength
Energy versus Frequency
Table 5.2 Two different ways of representing the energy of a light wave or photon.

In these expression the letter "h" stands for Planck's Constant, "c" is the speed of light, "f" is the frequency and l is the wavelength.

The Electromagnetic Spectrum

One of the great triumphs of 19th century physics was to understand that light is a traveling electric and magnetic disturbance. Figure 5.2 illustrates this idea. The red and blue arrows represent growing and then fading electric and magnetic fields. Any change in the one produces the other and together they from a wave traveling at the speed of light. For this reason light is considered to be an electromagnetic wave

Figure 5.2 Video clip illustrating the idea that light (and all electromagnetic waves) consists of a distrubance of traveling electric and magnetic fields.

Visible light consists of photons with a wavelength between 400 nanometers to 700 nanometers. But, photons of shorter or longer wavelengths bracket the visible region to form a continuum called the electromagnetic spectrum.

The electromagnetic spectrum is also an "energy spectrum" for photons.

Figure 5.3 Applet demonstrating light in either wave or particle view as a part of the electromagnetic spectrum.

Example 5.2 How could figure 5.3 be modified to show how energy of photons depends on position in the spectrum? Which photon has the higher energy: an ultra-violet photon or an infra-red photon?

Solution Photon energy increases with frequency - Figure 5.4 would be a suitable modification of the diagram which reflects this. Ultraviolet photons have higher frequency (shorter wavelengths) than do infrared photon hence they are higher in energy.

Similarily, blue photons are higher in energy than red photons - you can use the applet in figure 5.3 to demonstrate this.

  Figure 5.4 Rollover image showing how energy of photons increases toward the higher frequency end of the spectrum.

Spectral Windows

Earth's fragile atmospheric covering is essential to life. BUT - it does complicate the llives of astronomers! Figure 5.5 illustrates the concept of atmospheric spectral windows. Molecules (primarily nitrogen, oxygen, water vapour and carbon dioxide) have insatiable appetites for most parts of the elctromagnetic spectrum. There are gaps or "windows" however, through which Earth-bound astronomers can see the heavens!

Immediately prior to the Second World War a new "era" in Astronomy began. Americans Karl Jansky and (independently) Grote Weber began to "look" at the sky in the radio region of the electromagnetic spectrum. Advances in radar technology during the Second World War added to this and during the 1950's and 1960's the new and extremely sub-field of Radio Astronomy was born. In the 1960's, with the development of satellite technologies, astronomers continued to enlarge the range over which astronomers can view the universe.

Table 5.3 summarizes atmospheric absorbers and the part of the spectrum that they absorb.

Figure 5.5 Spectral windows and the appropriate technologies used to "see through" these windows. (Diagram courtesy NASA)

 

molecule

region blocked

H2O (water vapour)
infrared, short radio
O2 (Oxygen molecule)
short radio
CO2 (Carbon Dioxide) and other greenhouse gases
infrared
ozone
completely blocks UV and shorter
variable transparency due to dust and cloud
visible
Table 5.3 Molecules responsible for atmospheric absorption

 

A Gallery of Images of the Universe in Different parts of the Electromagnetic Spectrum

Table 5.4 summarizes the major regions in which astronomical observations are conducted. If an atmospheric window exists, such observations can be conducted from the ground, otherwise observations are conducted above the atmosphere using an orbitting staellite.

Gamma

The most energetic events in the universe will produce gamma rays. Our atmosphere is, fortunately, opaque to Gamma rays. Gamma ray observatories are, for this reason, often orbiting satellites. The HETE (High Energy Transient Explorer) is an example. Another way to observe gamma rays (from the ground) is to observe the effect that gamma rays have on the atmosphere when they are absorbed. The image on the right shows the remnants of a supernova (exploded star) imaged in gamma rays using a ground-based technique.

X-ray

The image on the right is a composite of an X-ray image of the core of the nearby galaxy M81 shown in blue. X-rays are very energetic photons produced by very high temperature gases. In this case it is believed that a supermassive black hole in the core of the galaxy is heating the gas to temperatures necesssary for the production of x-rays.

(click on image to get larger view)

Ultraviolet

Purple shades in the image of M81 are color-coded to represent where UV radiation is detected. UV is produced by high temperature gas often associated with extremely bright and hot stars.
:( X-ray: NASA/CXC/Wisconsin/D.Pooley & CfA/A.Zezas; Optical: NASA/ESA/CfA/A.Zezas; UV: NASA/JPL-Caltech/CfA/J.Huchra et al.; IR: NASA/JPL-Caltech/CfA)

Visible

This is a Hubble Space Telescope image of the galaxy M81 in the visible wavelength region.

 

 

 

 

 

 

(click on image for enlarged view)

Infrared

This is stunning infrared image taken with the Spitzer Space Telescope and shows the central part of the Rho Ophiucus star froming region. Star forming regions contain large amounts of dus tand warm gas which glow brightly in the infrared.

 

 

 

(click on image for enlarged view)

Microwave

This is a compsoite image showing the microwave emission from Hydrogen gas (coded in dark blue) and the visible wavelength emission from the nearby galaxy M51. Notice how much more extensive the system is when viewed in the microwave and radio region.

 

 

 

( Image courtesy of NRAO/AUI and Juan M. Uson)

Radiowave

Looking into the center of MilkyWay galaxy using the radio region of the electromagnetic spectrum.

Table 5.4 The universe in different regions in the electromagnetic spectrum.

 

 

 

 

 

 

 

 

 

 

 

 


To gain understanding of the dual-nature of light and the electromagnetic spectrum

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Richard Feynman was one of the greatest physicists of the past 50 years. Here is a famous quote of his concerning quantum theory: