--------- #1286 - What Makes Electromagnetic Waves into Light?
- When studying electricity and magnetism science measured the forces and the relationships between charges and distances of separation. These forces behaved very similar to gravity but much stronger. When electric permittivity and magnetic permeability were defined it was discovered that the ratio of their product was the speed of light. That is how science discovered that light was an electromagnetic wave.
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- When we look at the stars we are looking at electromagnetic waves. Light just happens to be that part of the electromagnetic spectrum our eyes are sensitive to. Our eyes are the result of evolutionary adaptation. The light just happens to be that part of the electromagnetic spectrum that is radiated strongest from the Sun and that range of wavelengths that pass through the Earth’s atmosphere the easiest.
- In most of astronomy what we see is black or white. When something emits or reflects all the colors it appears white to our eyes. If something absorbs all the colors it appears black. Gray is just somewhere in between. We see color when we can separate the wavelengths of light.
---------------------- 400 nanometers = blue
--------------------- 600 nanometers = yellow
--------------------- 700 nanometers = red
These colors are in this frequency range:
---------------------- 750,000,000,000,000 cycles per second = blue
--------------------- 500,000,000,000,000 cycles per second = yellow
--------------------- 429,000,000,000,000 cycles per second = red
- In astronomy we see black and white because we are working with low levels of light, our eyes are not sensitive enough to separate the colors and all the colors are combined.
- When we look at the Full Moon it is white on a black background. The light reflection from the Moon is 8 to 12 % depending on which direction you are looking from the Sun or from the Earth. The percentage of light reflected from an object is called the “albedo”.
Black paint has an albedo of between 5 and 10%. If you could move the Moon to an asphalt highway the two would closely match. The Moon only looks white because it is in the black background of space.
- Even the asteroid “Malhilde” that is blacker than tar at 3% appears white when photographed with the background of blacker space.
- Fresh snow has an abeldo of 90%. Saturn’s moon, Enceladus, is 93% so the surface appears as white as snow. The planet Venus’ abeldo is 67%. The reflection is due to the sulfuric acid in its atmosphere. The Earth’s atmosphere is 12% by comparison. Mercury is 11%. Jupiter is 50%. Mars is 16%.
- To further illustrate the tricks on the eyes take a black cat and a sheet of white paper. Sunlight is 450,000 times brighter than Moonlight. A sunlit black cat is 20,000 whiter that a moonlit white paper.
- The eyes are working with such a small part of the electromagnetic spectrum. It is so great to be in astronomy today because we have instruments to detect the entire spectrum and computers to translate that data back into colors we can see. It is hard for us to believe that we are looking at electric and magnetic energy propagating through space.
- The velocity of propagation depends upon the permeability and permittivity of space
-------------------- v^2 = 1 / permeability * permittivity
- To illustrate how light and electricity and magnetism are interconnected lets begin defining these terms:
-------------------- Permeability = 4*pi *10^-7 Tesla * meters / Amperes
- Permeability is the ratio of magnetic flux density to the magnetic field strength
------------------- Permittivity = 8.8 * 10^-12 Columbs^2 / Newton * meters^2
- Permittivity is the ratio of electric displacement in a dielectric medium to the Electric Field strength. It is the degree to which the medium can resist the flow of electric charge.
- The velocity of electromagnetic wave propagation = v
-------------------- v^2 = 1 / ( 4*pi *10^-7) * (8.8 * 10^-12)
-------------------- v^2 = 9 *10^16 meters^2 / seconds^2
-------------------- v = 300,000,000 meters / second
- This is the velocity of light.
- To further illustrate this we should define some of the units of measurement for permittivity and permeability. An Ampere is the current in 2 long parallel wires of length (L) that are 1 meter apart such that the attractive force is (2*10^-7 Newtons / meter) * (L)
- When the force between 2 charges separated by 1 meter is equal to 9 *10^9 Newtons the charge of each is 1 Coulomb.
- The Electric Force between charges varies as the inverse square of the distance between them.
--------------------- F = proportional * 1 / r^2
The Electric Force depends on the amount of charge on the two objects.
-------------------- F = proportional * q1 * q2
- When things are proportional they can be made into an equality with the proper Constant of Proportionality. “k” is the Constant of Proportionality.
-------------------- Electric Force = k * q1 * q2 / r^2 Newtons
--------------------k = 1 / 4 * pi * permittivity
-------------------- Permittivity = 8.8 * 10^-12 Columbs^2 / Newton * meters^2
- Note that this equation for Electric Force is very similar for the equation for Gravity Force.
----------------------- F = G *m1*m2 / r^2
- We can compare the two forces for the electron and the proton:
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-------------------- Gravity Force = (6.7*10^-11 Newton * meters^2 / kilograms^2) * (9.1*10^-31 kilograms) * ( 1.7^10^-27 kilograms) / (5 *10^-11 meters)^2
------------------- Gravity Force = 4*10^-47 Newtons
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------------------ Electric Force = k * q1 * q2 / r^2 Newtons
------------------Electric Force = (9*10^9 Newton * meters^2 / Coulombs^2) * (1.6*10^-19 Coulombs) * (1.6*10^-19 Coulombs) / ( 5*10^-11)^2 Newtons
----------------- Electric Force = 9 * 10^-8 Newtons
- The Electric Force is 2,000,000,000,000,000,000,000,000,000,000,000,000 times more powerful than the Gravity Force. Why is Gravity so weak?
- Electromagnetic waves propagate through non-metallic matter as well. We have to replace the permittivity and permeability of free space with that of the material but the math remains the same. Example: Teflon permittivity = 21 Columbs^2 / Newton * meters^2
- To further illustrate how light is tied to electricity and magnetism let’s define some more of the units for permittivity and permeability:
---------- Tesla is a unit of magnetic flux density defined as 1 Weber or magnetic flux per square meter. A Weber is a unit of magnetic flux from a circuit of one turn of wire producing an electromotive force of one volt when the flux is reduced to zero in one second. A Volt is the electromotive force between two points on a conductor carrying current of 1 Ampere when the power dissipated is 1 Watt. An Ampere is a current carrying a charge of 1 Coulomb per second. A Watt is a unit of power of 1 Joule per second ( 1 horsepower = 746 Watts ). A Joule is a unit of work done by a force of 1 Newton moving an object 1 meter. Work is the change in Kinetic Energy. Kinetic Energy is mass times velocity divided by 2.
- It all boils down to energy. That is how the Universe started. As it expanded and cooled the energy froze out in many different forms. Energy can not be destroyed it just changes from one form to another as the Universe grows in complexity.
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707-536-3272, Thursday, August 4, 2011
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