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.

Isaac Newton

courtesy courtesy

Christiaan Huygens

courtesy Images/people/Huygens.gif

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. 

Red light has a lower frequency than blue light. (animation)

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
visible Red light = .0007 mm  or 7000
Blue light = .0004 mm or 4000
x rays
gamma rays very short (a few ) very high

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

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:

Making a continuous spectrum

A continuous spectrum (the spectrum of white light)

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

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

(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 (λmax) 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 .  Using Wien's Law, you find:

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


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. 2023 - all rights reserved