Experiment IV-5

Energy Waves & Polarity

visible light, cosmic microwaves & a connection with gravity waves



The ancient Greeks considered what we now call energy (they called it fire) as one of the basic elements of the universe.  The alchemists adopted the Greek view of four distinguishable elements (earth, water, air and fire) composing all earthly materials.  Even two millennia later in 1789 when Antoine Lavoisier argued for a better definition of element, he continued to believe that light (Lumiere) and heat (Calorique) were material substances.

In Lavoisier's time cannons were manufactured by boring out a shaft from a metal casting, a process releasing much heat.  It was commonly thought that latent heat in the metal was liberated depending on the amount of material ground away.  But in 1789 Count Rumford, (birth named Benjamin Thompson, 1753-1814) while supervising boring cannon, noted the heat produced depended on the friction rather than the volume of removed metal.  (Read his original published report)  He proposed that heat could not be a material substance but must rather be a form of motion.  (Interesting trivia:  Marie Anne Paulze, married to and science partner with Lavoisier for 22 years until his execution, 11 years later had a much less successful marriage with Rumford.)


A century earlier Isaac Newton had used material particles to explain properties of light such as color, refraction and interference.  While several of his contemporaries, notably Robert Hooke (1635-1703) and Christian Huygens (1629-1695), suggested explanations which presumed light was composed of waves, most scientists of the 18th Century continued to believe that light was composed of particles of matter.  But with doubt being cast upon the assumption that heat was composed of material, the particle model of light also was rejected and replaced with Thomas Young's (1773-1829) view that waves better explained all the known phenomena of light.  Young also introduced the term energy to the quantity mv2 and to make work done (which he defined as force x distance) proportional to energy.  It took another decade of experiments by Augustin Fresnel (1788-1827) to convince the world that light is waves of energy.

wave types


Material can be disturbed by a wave in several distinct directions.  For example in sound, the material can be alternately compressed and rarified in a disturbance that expands outwards from the source.  The wave travels the same direction as the disturbance.  This is called a compression wave.

On the surface of a liquid (e.g., the ocean) or sheet (e.g., a flag) the material is displaced perpendicular to the surface.  For example the surface of the ocean goes up and down as the wave crosses the surface.  This is called a transverse wave.

Cohesive materials may be twisted (e.g., a lawn hose or a wire) with neighboring parts of the material receiving the twist a very short time later.  This is called a torsional wave.

Some materials can wave simultaneously in multiple modes (e.g., earth quakes).

wave polaritySome transverse waves have an additional feature.  While the surface of the ocean is forced to wave up and down, a violin string can vibrate in various directions.  The string might vibrate parallel to the surface of the violin or it might be caused to vibrate so the string gets closer then further from the violin.  Actual vibrations of the violin string could be any combination of possible directions.  But if a transverse wave vibrates in only one of multiple possible planes (say just parallel to the violin), then the wave is said to be polarized.

MaxwellJames Clerk Maxwell (1831-1879, at right) collected and developed mathematical equations to make Michael Faraday's qualitative insights about electricity and magnetism more useful.  Comparing the strength of electric interactions with that of magnetic interactions yielded a ratio with the units of velocity (3 x 108 meters/second).  Maxwell presumed this to be the speed at which the effect of a changing electric or magnetic field would spread away from the source.  Maxwell noted, This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations, if any) is an electromagnetic disturbance in the form of waves propagated though the electromagnetic field according to electromagnetic laws.  Maxwell improved on the agreement between these two speeds by experimentally comparing the attractions of two electric currents flowing in coils of wire (magnetic interaction), and the attraction or repulsion between two metal plates which have each received a charge of electricity (electric interaction).  Maxwell's electromagnetic theory, which accounted for all the then known phenomena of magnetism and electricity, was published in a semi-popular form in the Philosophical Magazine in 1861 and 1862.  According to Maxwell's theory, electromagnetic waves are transverse and can be polarized.


Iceland SparIn 1669 a transparent crystal from Iceland was sent Erasmus Bartholin (1625-1698) who noted that objects seen through it appeared double.  A century later Thomas Young suggested that two light rays polarized at right angles to each other must be refracted at different angles.  It is currently understood that such materials as Iceland Spar (sample at left) have electric charge distributions in their crystal lattices which slow light waves of polarity aligned with one dimension by a different amount than light waves with perpendicular polarity.  As a result half of the light is refracted at one angle while the remainder of the light is refracted in a different direction.  Notice light from the text is split different directions by the Iceland Spar making duplicate new and speak.  This provides evidence that light must be a transverse wave.

William Nicol (1768-1851) found that two crystals of Iceland Spar attached together could be used to measure the angle of polarization of compounds.  But such devices were very expensive.  Edwin Land, (1909-1991) when a Harvard freshman, conceived the idea that a comparable polarizer might be made by aligning tiny crystals (iodoquinine sulphate) embedded in transparent plastic, an idea which he patented in 1929.  Light of one polarity is absorbed while that perpendicular passes through.  Such film made by Land's Polaroid Corporation can be obtained in Polaroid sun glasses.  (Land and his Polaroid Corporation later invented cameras that produce self-developing photographs and ultra-sonic range finders used for self-focusing cameras.  Land trailed only Thomas Edison in the number of patents;  Land received 535.)

We shall use Land's polaroid filters to investigate polarized light.


Obtain the following materials:
  1. The sty on a clear day selectively scatters and transmits light of different polarity.  Look through a polaroid lense at the sky.  Rotate the lense looking for changes in the sky color and brightness.
    1. At what angle from the sun is the effect greatest?
    2. What angular rotation of the lense changes from minimum to maximum intensity?

  2. Reflections of surfaces such as water, glass and counter tops are often polarized.  Look at several reflective surfaces through a polaroid lense at the sky.  Rotate the lense looking for changes.  Look from several different locations so that the angle of reflection varies from perpendicular to a small grazing angle to the surface.
    1. At what angle to the surface is the effect greatest?
    2. Describe what varies when observing light reflected from a transparent surface such as a window?

  3. Many materials are optically active.
    1. Stack two polaroid lenses on top of each other, noting what can be seen through the lenses as one is rotated compared to the other.  (The lenses of SOME inexpensive sun glasses can be carefully popped free allowing the glasses to be restored after the investigation.)
    2. Place transparent materials such as clear tape partially between the stacked polaroid film.  Note some material may rotate the transmitted polarized light changing the intensity of light passing through the stack.  Other transparent materials may not be optically active.  Which materials are optically active?
    3. How are colored light effected by the optically active material?  (A collage of active material may produce interesting effects?)
    4. Investigate how a glucose or fructose sugar solution placed between the stacked polaroid filters effects transmission.

Microwaves and their role in the search for effects of gravity waves

uniform cosmic microwave backgroundIn 1963 P. J. E. Peebles and R. H. Dicke had calculated that if a Big Bang had occurred as some astronomers had proposed, residual energy must remain.  They were preparing an experiment to look for any such evidence when A. Penzias and R. Wilson accidentally discovered a background of 2 mm long electromagnetic waves, called microwaves.  (See Experiment II-10 for details.)  The universe has a uniform glow of microwaves in all directions as shown by the 360° image of the sky flattened at right.  This is direct evidence that billions of years ago the universe was very hot, but as happens to any expanding gas, the temperature has cooled and is now below 3K.

amplified cmb differencesIn February 2003 a team using a satellite called the Wilkinson Microwave Anisotropy Probe (WMAP) announced the first detailed full-sky map of the cosmic microwave background.  A detailed analysis of the radiation reveals systematic variations of about one part in a thousand caused by the earth's motion through the Milky Way galaxy.  The small differences have been amplified here so slightly warmer microwaves are shown redder in the direction the earth is moving and 180° away in the direction from whence the earth came microwaves appear a cooler blue.

cmb with earth's motion removedIn the third view, the variation in the microwaves due to the motion of the earth have been systematically eliminated.  Our relatively close Milky Way galaxy still contributes a warmer band across the sky.  But most of the rest of the light originated from the early universe shortly after the hot plasma had cooled enough to become transparent, releasing the light.  Here with the variations are magnified by about a million more than in the first image, the universe has a globular pattern.

cmb from ancient universeBy summing the cosmic microwave background at several different wavelengths, the warmth of the Milky Way has been removed leaving a detailed view of the tiny granular variations that were present in the early universe.  (Recall that while light travels fast, it still takes some travel time.  Light coming the greatest distance carries information from long ago.)  The distant granules visible in this image presumably eventually coalesced forming galaxies and stars similar to those now visible closer.  Analysis of the WMAP detected light from the distance past gives the following information about the universe:

This information has largely been confirmed by other independent measurements of data from quasars and distant type Ia supernovae.  But there should be more information embedded in the cosmic microwave background.

Because the microwaves are electromagnetic radiation, they can be polarized like the visible light investigated in this experiment.  Just like visible light becomes polarized by scattering, scattering also causes the cosmic microwave background radiation to be polarized.

In addition, gravity is believed capable of polarizing light.  If there were large gravity waves as expected of the early universe, it is believed they could create a distinct polarization of the cosmic microwave background with a tensor curl.  Several projects are now underway to search for the effects of early gravitational waves on the polarization of the cosmic microwave background.



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created 3/2/2004
revised 4/21/2004
by D Trapp
Mac made