The Physics of Light

Astronomers have a disadvantage in many respects because they are forced to observe the universe from a distance.  That is, much of the information they obtain originates from data which is "delivered" to them from the distant corners of the universe.  For example, they do not have the freedom to directly sample small pieces of a remote galaxy, an exploding star, or some of the planets (we are making some progress here).  Much of the information astronomers get comes from the light and radiation the various objects emit into space.  It therefore becomes very important to understand the nature of this radiation so we can dig out the clues it contains.

What is light?

In the 17th century two great scientists, Newton and Huygens, argued this point (to no definitive conclusion).  Newton argued that light was a particle and Huygens contended that it was a wave (much like the waves you find in water).   In 1801, Thomas Young showed, conclusively, that light behaved like a wave.  A little over a century later, quantum theory showed that light has properties of a particle.  As it turns out, modern physics has shown that light does, indeed, have properties of both waves and particles.

When Isaac Newton passed white light through a prism, it separated into a "rainbow" known as the visible spectrum.  Since one color blended into another without any gaps, this has also been called a continuous spectrum.

A continuous spectrum from white light

The wave nature of light -visible light and the electromagnetic spectrum

Light is a wave ... called electromagnetic radiation.  It consists of an oscillating electric and magnetic field that travels through empty space.  This radiation moves incredibly fast in a vacuum  ... 186,000 miles each second.  The speed of light is designated by the letter c and all the colors in the visible spectrum travel (in a vacuum) this same speed.  What, then, distinguishes one color from another?  One answer is the wavelength, designated by the Greek letter lambda - λ.  This is the distance between successive wave crests.  Red light has a slightly longer wavelength than blue light.

Red light (at one end of the visible spectrum) has a longer wavelength than blue light.  However, another way of distinguishing between the different colors of light is by their frequency, that is, the number of waves that pass by a point every second.  The animation below is designed to show that red light has a lower frequency than blue light. Imagine you were able to count the waves as they pass by the vertical line.  Since both red and blue light travel at the same speed, you would find more blue waves passing the vertical line each second than red waves because the blue waves are closer together. 

The relationship between frequency (f) and wavelength (λ) may be expressed in the following equation:

where c is the speed of light.  Notice that as wavelength increases, frequency decreases.

But there are waves with higher frequencies (shorter wavelengths) than blue light and waves with lower frequencies (longer wavelengths) than red light.  These radiations are invisible to the eye but exist in nature.  Together they form the electromagnetic spectrum.
 
 

Wave type	Wavelength	Frequency
radio	very long - several meters	very low
microwaves
infrared
visible	Red light = .0007 mm  or 7000 Å
	Blue light = .0004 mm or 4000 Å
ultraviolet
x rays
gamma rays	very short (a few Å)	very high

Note: Å stands for angstrom units.  An angstrom is a very small distance.  1 meter = 10,000,000,000 Å

Infrared radiation

Just beyond the visible spectrum there exists electromagnetic waves which have slightly longer wavelengths than red light, and are known as infrared radiation.  Although we can't see this radiation, we can feel it in the form of heat.  On a warm summer day you can feel the warmth of the sun on your skin.  This is coming from solar infrared radiation.  Your own body radiates quite a bit of infrared radiation, which can be detected by "heat scopes".  This is often depicted on TV and in the movies where someone can spy on people in the dark with infrared goggles.  One of my favorite movies, Predator, tries to convey the fact that the alien is adapted to view an infrared universe (not a visible one).

Your instructor in infrared (an improvement over the visible one)

Ultraviolet radiation

With a wavelength just slightly shorter than violet, ultraviolet radiation is also invisible to our eyes.  Everyone should be aware that the sun emits UV and that wearing UV blocker will protect you from this harmful radiation.  All sunburns are caused by UV damage to your skin cells.  In fact, you avoid any types of radiation with short wavelengths ... especially prolonged exposure to X Rays and certainly Gamma rays (how do you think the Hulk was made?)

Some basic physics

Here are some main points you need to understand:
 

• Light behaves like a wave ... the color we see depends on the wavelength.
   
  • White light is actually all the colors of the visible spectrum combined
    • a prism can separate the light into the separate colors ... in nature, a raindrop provides the prism to make rainbows.
    • another device, called a diffraction grating can do the same thing.
       
  • Red light has a longer wavelength than blue light.
    • red light has a wavelength of only .0007 mm.
    • blue light has an even shorter wavelength ....  .0004 mm.
    • the width of a wire that makes up a paper clip is about 1 mm.
    • red light has a lower frequency than blue light.
       
  • There are other wavelengths of "light" beyond the range of our eyes (visible).
    • infrared, microwaves and radio waves have even longer wavelengths.
    • ultraviolet, X rays, and gamma rays have very short wavelengths and are dangerous to humans

Making a continuous spectrum

• If a solid (or a compressed gas) gets hot enough, it will convert some of that heat into light in a process is known as incandescence. In fact, it will emit a continuous spectrum of light (all the colors from red to blue) ... and also some energy in the "invisible" infrared and ultraviolet.  You can see this for yourself by passing white light through a prism.  A raindrop is a natural prism which lets you see the spectrum of the sun as a rainbow.  This is kind of a "brute force" way of making light.

 

• An incandescent light bulb uses electric resistance to produce heat which makes light.  Thomas Edison tried thousands of different filaments which would stand up to the heat in his quest to invent the light bulb.  Actually an ordinary incandescent light bulb will emit quite a bit of its energy in the infrared (IR) and some in the ultraviolet (UV). 
   
• A physicist named Max Planck laid down the theoretical framework for radiating objects.  He considered ideal conditions in which an object did not interact with other matter and was in a constant state of equilibrium.  In these cases, he was able to show exactly how a perfect radiator would behave at any temperature.  This theoretical object was known as a blackbody.  Although nothing really behaves in this perfect state, many objects (like stars) come very close, so much can be learned by studying the behavior of a blackbody.
   
• Below is a blackbody radiation curve similar to the one produced by the sun.  Note: The outer sun (photosphere) is not a solid but a hot compressed gas ... which also produces a continuous spectra. 




Radiation Curve of the Sun's Photosphere (a hot compressed gas)

Notice that the sun produces roughly the same intensities of all the spectral colors.  This is why the sun looks "white" to the eye ... (do not look directly at the sun).  Also note that the sun produces quite a bit of radiation in the "invisible" ultraviolet and infrared.

 

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Playing with Blackbody Radiation curves

I've found a great source on the web that lets you examine the radiation curves of blackbodies at different temperature.  Please spend some time with it so you can discover some important properties of radiators.  If nothing appears below, please click on this link.

 

Please pay attention to two changes as you adjust the temperature - the overall intensity and how the "peak" shifts.

After playing around with the applet above, you will learn some important properties of radiators. They are:
 

• The hotter the object becomes, the more energy it radiates (per unit area) each second. That is, intensity goes up (the curve gets bigger)... at every wavelength. This has been summarized in a law known as the Stefan-Boltzmann Law. It says that the energy output of a radiator increases as the temperature increases according to this equation. (T is in Kelvin -see below)
  
  
  
  Simply stated, the hotter, the brighter!
   
• As the temperature increases, the "peak" radiation shifts toward shorter wavelengths. That is, cooler objects put out more radiation in the infrared. Hotter objects put out peak radiation in the visible. Really hot objects produce their maximum intensity in the ultraviolet.
  
  
  
   
• This has been summarized in a law known as Wien's Law.

(T is in Kelvin -see below, λ is in angstroms )

This also gives astronomers a great way of determining exactly how hot  a star's photosphere is.  By rearranging this equation, you find:

All you need to do is scan the spectrum of a star and determine where it emits the peak radiation (λ_max) and apply the formula.

Example:  An astronomer scans the spectrum of a star (below) and maps the radiation curve (shown in red).  What is the temperature of this star's photosphere?  Measurements show that the star puts out the most radiation at 3986 Å.  Using Wien's Law, you find:

T = 28,900,000 / 3986 = 7250 K

•  This also means that the color we see is a function of temperature.  Remember we can only detect the visible with our eyes, so we are not "seeing" the whole story.  Go back to the blackbody simulator and choose a low temperature around 3000K.  Now look only at the intensities in the visible.  You will discover that there is a lot more red light produced than blue light.  This means we see a cool star as red.
  
  Now try again with a higher temperature around 5500 K.  You will discover that all the colors in the visible are roughly equal in intensity.  This is white light.  Note: our sun is considered a "white hot" star.  It appears yellow because our atmosphere filters out much of the bluer light. 
  
  Finally, crank the temperature up to 8700K or "A".  You will see that the star puts out much more blue light than red, ... making it appear blue in color. 

• Cool stars appear red, hot stars appear blue!  (So forget the phrase "red hot")

Kelvin temperature scale

You may have been confused by seeing all the temperatures listed in K.  What is a K???  To honor Lord Kelvin, a temperature scale was created where zero point is absolute zero.  Absolute zero is the coldest anything can get and it is very cold ... -460 Fahrenheit or -273 Celsius.  Don't worry too much about this, but get used to seeing all temperatures listed in Kelvin.  Deep space (space far from any stars) is very cold ..  only 3 Kelvin.  You thought Wisconsin was cold?

Cosmic Rays

Before we finish this section, we need to toss in one more item - cosmic rays.  Until now, all forms of radiation were electromagnetic in character.  That is, a wave of energy that can travel through space at 186,000 mi/sec in a vacuum.  Space is also a host for high speed particles which can travel near the speed of light, and under the right conditions, be just as deadly as large doses of x-rays or gamma rays.  Cosmic rays can consist of fast moving electrons, protons (mostly), or even nuclei of helium atoms (known as alpha particles).  The source of these particles are extremely energetic events (coronal mass ejections on the sun, exploding stars, bipolar flow from black holes, etc) ... but fortunately our magnetic field and atmosphere acts as a shield (or buffer) from this "buckshot" from space. 

©Jim Mihal 2004, 2014- all rights reserved