Astronomy 100 -- Light and Radiation


White light is the combination of all colors. When the colors are spread out, as in a rainbow, it is called a spectrum.
What colors, you ask?
Red, Orange, Yellow, Green, Blue, Indigo and Violet.

Light is also called radiation, and it all travels at the speed of light, c, which is 300,000 km per second!

Light is a form of energy!

Light has the properties of both a particle and a wave. Light particles are called "photons". The amount of energy a photon has depends on it's wavelength (ah, a wave property!).

In addition to visible light, there are various forms of "light" we cannot see. Listed below are the various types of radiation, from the highest energy photons to the lowest energy photons.

These various forms of light, or radiation, make up the


How Wavelength and Energy are Related in the
Electromagnetic Spectrum

Radio Micro InfraRed Visible UltraVioletX-RayGamma Ray
Long --> ---> Wavelength ---> --> Short
Low --> ---> Energy ---> --> High

Visible light waves have very small wavelengths. A common unit used to measure these wavelengths is the Angstrom.

1 cm = 100,000,000 Angstroms

Visible light has wavelengths between 4000 A (violet light) and 7000 A (red light). Note that since violet light has a shorter wavelength than the red light, the violet photon has more energy than the red photon.


Temperature is a measure how much heat energy an object (such as an atom or molecule) has. The heat energy is related to how fast these atoms or molecules are moving or vibrating. The hotter an object gets, the faster the atoms or molecules will be moving or ibrating.

A molecules in a solid can be pictured as a bunch small balls in a crystal pattern attached to the neighboring molecules by springs. When heat is added, the molecules vibrate pulling on these springs. The hotter they get, the more they will stretch and pull on these springs. When enough heat is added, they can break the springs and when this happens, a solid becomes a liquid.

In a liquid, the molecules are free to move around randomly, but they are still close to other molecules and bounce off of them. By adding more heat energy, these collisions become very hard. When enough heat is added, the collisions be hard enough that some of the molecules free themselves from the liquid. These molecules are now in a gaseous state.



As heat is added to an object, the molecules move faster. So, as heat is removed the molecules will move slower.
Image continually removing heat from an object. The molecules will continue to move slower and slower. At some point, a temperature will be reached at which the molecules will stop moving. This temperature is the lowest temperature possible and is know as ABSOLUTE ZERO! This turns out to be -273° C.

This is an excellent starting point for a temperature scale and is the starting point for the Kelvin temperature scale.

Absolute Zero = 0 K

Celsius (or Centigrade) temperatures can be converted to Kelvin temperatures simply by adding 273 to the Celsius temperature.

TKelvin = TCelsius + 273

As Absolute Zero is the starting point for the Kelvin scale, there are no negative temperatures in the Kelvin scale.


      Scale    Water Freezes    Water Boils
   Fahrenheit            32 °          212°
   Celsius              0°          100°
   Kelvin          273°          373°


      Object        Temperature
   Absolute Zero                              0 K
         Room Temperature                          295 K
   Surface of Venus                          750 K
   Surface of the Sun                        5700 K
   Solar Corona                2,000,000 K
   Core of the Sun              15,000,000 K
   Core of a Dying Star            200,000,000 K
   Universe 1/100 Second Young        100,000,000,000 K



More about WIEN'S LAW

Wien's Law shows there is a relationship between the temperature of a hot object and the peak wavelength of emission, or the color at which the most energy is emitted from the object.

Wien's Law can also be written as

As the temperature of an object increases, the peak wavelength will decrease to shorter wavelengths.

While temperature must be measured in Kelvin, the peak wavelength can be measured in various length units, which then determines the valve of the constant to be used.

If the peak wavelength is measured in nanometers, the constant has value of 3,000,000 nm K (For complete accuracy, the value is 2,900,000 nm K). If the peak wavelength is measured in microns, then the value of the constant is 3,000 micron K.

See some examples here.

So, for any object emitting a continuous spectrum, we can determine its temperature just by measuring the wavelength of peak emission (what color is most of the light).


In the form of a heated gas, each element produces a very SPECIFIC and UNIQUE emission line pattern, much like a fingerprint of the element. This unique pattern allows the different gases in a mixture to be identified.

This is NOT TRUE for the sources of a continuous spectrum.

Gases can only absorb the same colors they emit!

The hydrogen emission line spectrum.

The hydrogen absorption line spectrum.

Note the same colors are emitted and absorbed!

The Mystery of Emission-Line Spectra
Atomic Absorption and Emission Spectra
Examples of Emission-Line Spectra


The Stefan-Boltzmann Law allows us to determine how much energy comes from a given area, say 1 square meter, of an object that emits a continuous spectrum. How much energy is emitted from this given area depends only on the temperature of the object! All objects (be it silver, iron, lead) that produce a continuous spectrum when heated will emit the same amount of energy if they have the same temperature. If we can determine the temperature for an object (maybe using Wien's Law?), we can then determine how much energy is emitted from each square meter of the object.

Note: The emitted energy goes as T (temperature) to the 4th power! So if I make T (the temperature) 4 times larger, the energy emitted from each square meter increases by 4 x 4 x 4 x 4 = 256 times!

How can we make an object emit more energy?

Outer Moons and Icy Bodies.

The Nearest Star: The Sun.

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