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--------------------- 2492 - ELECTROMAGETIC SPECTRUM - how we see?
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- What Do You Think is the Oldest Thing You Can See? It is light itself. It is the Electromagnetic Spectrum.
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- If a tree falls in the forest and there is no one there to hear it, does it make a sound?
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- If light travels for billions of years through space and does not strike the retina of the eye stimulating the sensation is it still light?
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- Light is electromagnetic radiation, interdependent, mutually perpendicular transverse oscillations of electric and magnetic fields. Wow, that is a mouth full. It appears that light is not simple.
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- The spectrum of light is defined by the wavelength of a single cycle of this oscillation. The spectrum of light that our eyes can see stretches from violet light at 400 nanometers wavelength ( A frequency of 750,000,000,000,000 cycles per second ) to red light at 700 nanometers wavelength ( A frequency of 428,000,000,000,000 cycles per second ).
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- Above the frequencies of violet light is the ultraviolet and below the frequencies of red light is the infrared. The table below shows that light is but a small part of the total electromagnetic spectrum:
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---------------------------- Wavelength, meters
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Gamma Ray 0.0000000000001
X-ray 0.00000000001
Ultraviolet 0.00000001
light-blue 0.0000004 400 nanometers
light-red 0.0000007 700 nanometers
infrared-near 0.000001
infrared-far 0.001 one millimeter
Cosmic microwave background 0.01 one centimeter
Microwaves 0.1 ten centimeters
radio - UHF TV 0.6 60 centimeters
radio - VHF TV 1.5
radio FM 3
radio AM 300 328 yards
radio low frequency 299,800 186 miles
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- The eye cannot detect radiation that is less than 300 nanometers wavelength on greater than 700 nanometers wavelength. We can not see ultraviolet and infrared but we can feel it. Ultraviolet radiation gives us suntans and infrared radiation makes us feel warm. Note that the 1.5 meters is the size of a TV antenna.
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- The atmosphere’s ozone layer absorbs ultraviolet radiation that is less than 300 nanometers. Therefore, ultraviolet astronomy can only be carried out using satellites and balloons that get above the stratosphere.
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- This table converts wavelengths to nanometers ( 10^-9 meters) and shows the frequency in cycles per second:
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------------------- Frequency Wavelength
- Electromagnetic cycles/second Nanometers
- Spectrum meters * 10^-9
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- Gamma Ray 3.E+21 0.0001
- X-ray 3.E+19 0.01
- Ultraviolet 3.E+16 10
- light-blue 749,481,145,000,0 400
- light-red 428,274,940,000,00 700
- infrared-near 299,792,458,000,0 1,000
- infrared-far 299,792,458,000 1,000,000
- Cosmic background 29,979,245,800 10,000,000
- Microwave 2,997,924,580 100,000,000
- radio - UHF TV 500,000,000 599,600,000
- radio - VHF TV 200,000,000 1,499,000,000
- radio FM 100,000,000 2,998,000,000
- radio AM 1,000,000 299,800,000,000
- radio low frequencies 1,000 299,800,000,000,000
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- The sky is blue because the shorter wavelength light ( blue, 400 nm ) is scattered by the molecules in the air more that the longer wavelength light ( red, 700 nm ). Based on the 1.7 ratio of red wavelength to blue wavelength blue is scattered 8 times more than red light. You see the blue 8 times more abundantly than you see the red light when you look up in to the sky. For the same reason you see the Sun is yellow. What is the true color of the Sun?
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- The Sun is not actually yellow, it just looks yellow. This is because sunlight coming through our atmosphere is scattered more strongly in the violet and blue wavelengths than the green, yellow, orange, and red wavelengths. This scattering affect shifts the peak of the Sun’s intensity entering your eyes from blue-green toward yellow.
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- The Sun is emitting all the colors, but its peak intensity is at 500 nanometers corresponding to a surface temperature or 5800 degrees Kelvin.
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- Peak Wavelength = .0029 / temperature
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- 500 nm = .0029 / 5800 Kelvin
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- 500 nanometers is light that is blue-green in color. When light travels 12 billion years and strikes a parabolic mirror in a telescope before it strikes the retina of the astronomer’s eye it is already 12,000,000,000 years old. This light is older than Earth itself.
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- In addition to the atmosphere creating the yellow color, the evolution of the eye over millions of years has made the eye most sensitive to yellow light. That is why some learned educators have suggested that we have yellow fire trucks instead of red ones.
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- You have heard the expression, “ Grandpa is older than dirt “. If you count dirt as Earth it is only 4,500,000,000 years old. The light from the galaxy in the telescope started on its journey before the Earth was formed in our solar system.
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- The astronomer is a archeologist looking backward in time. The galaxy that she is seeing is as it was 12 billion years ago in its early childhood. It may have disintegrated into a black hole by now. Who knows? We may have to wait another 12 billion years to find out.
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- Looking into the sky is looking into our past. The oldest thing you can see in the night sky with the naked eye is the Andromeda Galaxy. This light reaching your eye is 2,200,000 years old. By studying more distant galaxies using telescopes astronomers can learn about the formation and early childhood of our Universe.
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- There is something even older that you can see and hear by tuning between the stations on your television. The static hiss you see and hear in those older TV’s is for the most part the cosmic microwave background radiation that first broke free after the Big Bang. This radiation is 14,000,000,000 years old.
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- The radiation first broke free from the Big Bang plasma when the Universe was 1 % its current size. It had expanded and cooled to 3000 degrees Kelvin. Cool enough for the ions and electrons to combine into hydrogen and helium atoms.
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- This neutral plasma allowed the light photons to escape and radiate into the Universe. Today this radiation has cooled to 2.725 degrees Kelvin. It was born just 400,000 years after the Big Bang. That is the oldest thing we can see.
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- It was not until the Universe had expanded to 20% its current size that gas clouds could form into our first galaxies and stars were born. Today the Cosmic Background Radiation represents 99% of all the radiation in the Universe, and it only has a temperature of 2.3 degrees Kelvin above Absolute Zero.
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- All electromagnetic radiation comes to us in small quantum packets, called photons. Photons can behave both as waves and as particles. All photons with the same energy are identical.
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- Their energy depends solely on their frequency of oscillation. The hotter any object becomes the shorter the wavelength it emits. An heated iron rod turns red, then orange, then yellow, then white hot. This relationship is shown in the table below:
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- Energy = constant * frequency
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- The constant is Planck’s Constant = 6.626076 * 10^-34 joule * seconds
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- Energy Energy Temperature
- Joules Electron Degrees
- Electromagnetic Spectrum Volts Kelvin
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- Gamma Ray 2.00E-12 12,481,996 29,000,000,000
- X-ray 2.00E-14 124,820 290,000,000
- Ultraviolet 2.00E-17 125 290,000
- light-blue 5.00E-19 3.1 7,250
- light-red 2.86E-19 1.8 4,143
- infrared-near 2.00E-19 1.2 2,900
- infrared-far 2.00E-22 0.001 2.9
- Cosmic background 1.88E-22 0.001 2.725
- Microwave 2.00E-24 0.00001 0.029
- radio - UHF TV 3.34E-25 0.000002 0.005
- radio - VHF TV 1.33E-25 0.0000008 0.002
- radio FM 6.67E-26 4.E-07 0.001
- radio AM 6.67E-28 4.E-09 0.00001
- radio low frequencies 6.67E-31 4.E-12 0.00000001
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- From the table you can see that the energy of red light is 1.8 electron volts and the energy of blue light is 3.1 electron volts. Since one electron volt = 1.60210 * 10^-19 joules, this is equivalent to 2 to 5 * 10^-19 joules of energy.
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- For comparison, a 25-watt light bulb emits 25 joules each second.
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- So each photon does not have much energy. But, if you add up all the Sun’s photons that strike the Earth each second it totals 1370 watts per square meter.
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- No wonder you can get sun burn.
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- Other Reviews available to learn more include:
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- 1288 - We are immersed in electromagnetic radiation. How many photons are hitting you from a 60 watt light bulb?
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- 1286 - What changes electromagnetic energy into light? It is how the Universe started and the energy can not be destroyed. It can only change forms.
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- 1043 - The strangeness of light.
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- 726 - Electric and magnetic forces. And, the ratio compared to the force of gravity.-
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- 621 - Background radiation.
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- November 22, 2019. 2492 36
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--------------------- Friday, November 22, 2019 -------------------------
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