Spectroscopy Notes

Electromagnetic Spectrum

The electromagnetic spectrum includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x rays, and gamma rays. Visible light, which makes up only a tiny fraction of the electromagnetic spectrum, is the only electromagnetic radiation that humans can perceive with their eyes.   (MS Encarta 1998)

     Radio/TV and microwave waves are very big.  They have a wavelength of several meters and even kilometers (and even bigger than that too!).  Your eyes cannot see radio waves, but they enter and exit your house, your car, your body…etc at all times.  Microwaves in your microwave oven are at a wavelength which will make water molecules vibrate so much that they heat up (causing your food to get hot.  No water in the food would mean a microwave oven would be useless).

     Visible light has smaller waves than those of radio waves.  Visible light has a wavelength of 400 to 700 nm.  Visible light can be remembered by the acronym ROY-G-BIV.  Red has the longest wavelength at 700 nm and violet has the shortest at 400 nm. 

      Of course, Visible light can be blocked by solid objects and it can be bent by liquids and gases such as air and water.  (look at a pencil in a glass of water, it appears bent because the light going through the water is slowed down by the water and makes the pencil appear “bent” to your eye.)  The sky is blue because the molecules in our air bend the light from the sun so that blue light is scattered and the other colors come shining through to our eyes. 

      The sun may appear yellow or even red during certain times of the day, but this is due to “color subtraction” from our atmosphere.  The sun itself is white if seen from outer space where there is no atmosphere.

     X-rays, Gamma rays have wavelengths even smaller than those of visible light.  They are down in the picometer range (and smaller than that too!).  These rays have harmful effects on the human body.  Our atmosphere prevents many of these rays from reaching us (only light and radio waves get through

What happens when you heat or electrically excite a vaporized atom?

(eg.  flame test of copper sulfate or electric current through neon gas)

When a substance is vaporized, and the vapor is heated until it emits light, a single color may predominate, as in the yellow color of sodium-vapor lamps, the red color of neon lamps, and the blue-green color of mercury-vapor lamps. The spectrum in such cases consists of several lines of specific wavelength, separated by regions of absolute darkness. In the case of sodium vapor, the yellow color is produced by two lines of approximate wavelength 589.0 and 589.6 nm. The difference in color between these two lines is not detectable by the human eye, but the lines may be readily resolved, or separated and distinguished, by a good spectroscope. These two lines are called D2 and D1   (MS Encarta 1998)  This kind of spectrum is called an emission spectrum and can be seen below)


  Characteristic Spectra (above) Every chemical element has a characteristic spectrum, or particular distribution of electromagnetic radiation. Because of these “signature” wavelength patterns, it is possible to identify the constituents of an unknown substance by analyzing its spectrum; this technique is called spectroscopy. Emission spectrums, such as the representative examples shown here, appear as several lines of specific wavelength separated by absolute darkness. The lines are indicative of molecular structure, occurring where atoms make transitions between states of definite energy. 

 What happens when you heat that atom to an exceptionally hot temperature? 

 (eg.  The Sun or a Light bulb)

The simplest form of spectrum, called a continuous spectrum, is emitted by a solid object that is heated to incandescence, or by a liquid or a very dense gas. Such a spectrum contains no lines because light of all colors is present in it, and the colors blend continuously into one another, forming a rainbow-like pattern. A continuous spectrum can be analyzed only by spectrophotometric methods. In the case of an ideal emitter, a blackbody, the intensities of the colors within the spectrum depend only on the temperature. As the temperature is raised, the spectrum of blackbody radiation is shifted toward the higher frequencies in direct proportion to the absolute temperature. (MS Encarta 1998).  Cooler blackbodies are red.  Hotter are blue.  Try this experiment:  Use a dimmer switch on a light in your house.  Watch the color turn red-yellow-white

Absorption spectrum of stars:

An interesting phenomenon, however, occurs when you look at the spectrum of the sun.  Remember that the sun is white and therefore it emits all forms of electromagnetic radiation.  This means that it emits (among other things) the complete visible spectrum from 400nm to 700nm (ROY-G-BIV).  But, there are some “black lines” on the spectrum of the sun 

(see spectrum below).

These black lines indicate elements present in the atmosphere of the sun which are absorbing some of those wavelengths of radiation.  Remember that Sodium emits light at 589.0 and 589.6 nm on the electromagnetic spectrum?  Well, it also absorbs light at those wavelengths too.  You can see the two bands at “D”.  These bands represent the sodium that must be present in the Sun’s atmosphere and are absorbing the light.


Spectrum of the Sun (above) Radiation from the sun is photographed using a spectrometer and is analyzed through the use of a spectrograph. The dark lines in the spectrum are called absorption lines and are caused by the absorption of radiation by elements in the sun’s atmosphere. By studying these absorption lines, scientists are able to identify the elements present in the sun. The prominent line at the red end of the spectrum is one of the hydrogen lines, and the lines in the yellow indicate the presence of sodium.

Spectroscopy and Astronomy:

Astronomers use spectroscopy and these “black lines” to tell what elements are present in a star.  Stars should give off a continuous spectrum; but they don’t.  The black lines indicate which elements are present in their atmospheres. 

     On earth, we can take elements and heat them up with a flame and get these “black lines” to come out as colored lines.  That is how we know which elements are in the stars.

      In other words, if we heat up sodium on a Bunsen burner, we are going to see two yellow lines in a prism.  If we take a spectrograph of the sun, those same two yellow lines are going to be missing from the continuos spectrum.  This is because the sodium in the sun’s atmosphere absorbed those two yellow lines.

Cool facts about stellar astronomy:

1.  The element Helium was discovered using spectroscopy.   French astronomer Pierre Janssen discovered helium in the spectrum of the corona of the sun during an eclipse in 1868. Shortly afterward it was identified as an element and named by the British chemist Sir Edward Frankland and the British astronomer Sir Joseph Norman Lockyer. The gas was first isolated from terrestrial sources in 1895 by the British chemist Sir William Ramsay, who discovered it in cleveite, a uranium-bearing mineral.  Natural gas, which contains an average of 0.4 percent helium, is the major commercial source of helium.  The largest producer of Helium in the world is Texas. (MS Encarta 1998)

  Between 1973 and 1980, the natural gas companies were not required to recover helium from their pipelines so they let it go (into the air and thus into outerspace).  Then, in 1980, scientists convinced the government to reestablish a national helium reserve.  Helium is used in a variety of places where an non-combustible gas is necessary as a propellant or a cushion (eg.  pushing hydrogen and oxygen fuel out of the space shuttle.  Cleaning air hoses for deep sea diving…etc). Despite these conservation efforts, if present demands for helium continue the United States will exhaust its helium-rich reserves by the year 2016

2.  If a star is moving away from the earth, the “black lines” on the spectrograph move towards the red end of the spectrum.  This “red shift” or “Doppler effect” is how astronomers can tell how fast a star is moving away from us or towards us.  A star moving towards us shifts toward the blue end of the spectrum.

     As a star moves away, the wavelength of its emitted light is stretched.  Remember that red is the most stretched wavelength.  As a star moves closer, its emitted light is squished.  Remember that blue is the shortest wavelength.  (The Doppler effect can also be noticed by the sound of an approaching train, siren or car and then be noticed by its change in pitch as it passes you).

3.  If a star is very hot, then the molecules of gas inside of it will be moving very fast.  This makes those “black lines” of absorption spread out a little.  Astronomers can tell the relative temperature of stars by looking at these spreading “black lines” of absorption.  For example, the D1 and D2 yellow lines of sodium on our sun would be much broader on a star which is hotter than our sun.