History of Electromagnetic Radiation
Energy Escapes in the form of Electromagnetic Radiation
Incredibly, the sun only makes gamma radiation, yet x-rays, ultraviolet, visible,
infrared, microwave, and radio waves are all emitted from the sun. The interior
of the sun is so dense, that the gamma rays cannot escape without colliding
with atomic particles and losing a small portion of their energy. Traveling
at 300,000 km/sec, a gamma ray typically spends 30,000 years colliding with
atomic particles and re-emitting energy at a slightly lower energy level, until
it finally escapes from the sun and heads to earth. The gamma ray now has lost
so much of its initial energy that it appears as visible light, still traveling
at 300,000 km/sec, but now only with energy between 400 and 700 nm. This is
the light of the sun which we see as "visible light." Gamma radiation
which may escape more quickly will leave the sun with higher energy levels as
ultraviolet or x-radiation, while those gamma rays which take longer to escape
after surviving more collisions will escape as infrared, microwave or even radio
The sun ONLY makes gamma radiation, but due to time and chance collisions,
all forms of radiation are released. To be sure, it is more complicated than
this, but I am trying my best to keep the course at a high school introductory
level. I even find complete understanding of the energy interactions with electrons
on the escape path to be exceedingly challenging. I am getting better at understanding
this actual events, so if you want more knowledge or a place to go to learn
more, contact me :)
Absorption and Emission Spectra
here, we enter to one of my favorite parts of the course ... the history of
the spectral analysis of the Sun. Isaac Newton is credited with the explanation
for the splitting of light into the color spectrum by a prism during the 1600's.
In the 1700's, a young Austrian orphan named Joseph
Fraunhofer (left) found himself alone and homeless when his orphanage collapsed.
Most of the children were killed in the tragedy, and Joseph suddenly had nowhere
to live. A wealthy man found the young teenager and took pity on him, offering
a small bag of coins to get residence and a connection to a glass-making shop.
Joseph quickly demonstrated his talents for glass-making and moved from apprentice
to master status, becoming one of Europe's finest craftsmen. One of Joseph's
specialties was etching. He would scrape the glass without breaking it, resulting
in a mixture of clear and more opaque contrast within a piece of glass. One
evening, Joseph noticed that the sunlight passing through his etched glass was
broken into the corresponding color spectrum described by Newton. As he examined
the "rainbow" more closely, he noticed little dark lines, barely perceptible,
throughout the rainbow. Not knowing what they were or their cause, he set up
a little experiment to learn more about these lines. First, he made a detailed
map of the dark line locations, serving as a baseline for comparison to other
glowing objects. Then, by allowing moonlight to pass through one of his etched
glass pieces, Fraunhofer found dark lines in the rainbow from moonlight in the
exact same locations as in the solar rainbow. When he passed telescopic light
from Jupiter and Venus through the etched glass, the Jupiter and Venus rainbows
had the same dark lines in the same locations as the solar and lunar rainbows.
BUT, when he passed starlight from Sirius through his telescope and etched glass,
he found a rainbow with dark lines, but in different positions than the solar
rainbow. Fraunhofer discerned two things that evening. Moonlight, as well as
light from Venus, Jupiter, and other planets are mere reflections of light from
the Sun. The Moon and planets do not make their light, but reflect light from
the Sun. Meanwhile, others stars must also be making their own light, but each
star seems to make somewhat different forms of light.
Today, we know that these dark lines are called "absorption lines,"
and Fraunhofer's etched glass is called a diffraction grating. When an electron
in any atom is excited up to a higher energy orbital, it absorbs a photon of
a specific energy --- that energy described by the difference between the two
orbitals. The Sun's outermost layers and atmosphere are made of a large variety
of atoms. If enough electrons of those atoms absorb photons at a given energy,
an absorption line is formed. Given enough energy, electrons can jump more than
one level at a time. Fraunhofer was looking at the absorption lines created
by a light source shining through a gas, with the gas atom electrons absorbing
light energy from behind it. We call this spectrum an Absorption Spectrum. The
particular absorption spectrum for Hydrogen is found below. The two main lines
are in the red at 656.3 nm (H alpha) and in the blue at 486.1 nm (H beta). We
also know now that the many other lines in the solar absorption spectrum are
due to the presence of different ions of the same element and different elements
burning behind the gas envelope of the Sun.
Fraunhofer did little with his absorption line work, returning to the more
lucrative efforts from making glass in pretty forms. In the mid-1800's Robert
Bunsen was tinkering with chemistry experiments in Germany when he made
an interesting discovery. Yes, Bunsen is the same person who invented the Bunsen
Burner that you all enjoy so much in Chem labs, and are repeatedly warned to
never use in the presence of Ether, nor to roast tiny colored marshmallows in.
Bunsen was igniting elemental gases in vacuum tubes and looking at their colors.
This proved to be the beginning of "Neon Lights," for different gases
glowed with different colors. Bunsen one day looked at the glowing gas through
a diffraction grating and whoa! was he surprised. He did not see a nice continuous
spectrum of the rainbow as expected, but instead saw individual bright lines.
Every element that he burned and then looked at with his diffraction grating
generated a different set of bright lines. Bunsen discovered that these bright
line spectra were descriptive and unique to each element.
Today we know that when an excited electron drops to a lower energy orbital,
a photon is released. The result is an emission line (seen below), and the particular
emission spectrum for Hydrogen is shown below. Sending an electric current through
the vacuum tube filled with the trapped elemental gas causes the electrons to
jump to a higher energy level, and upon returning to the resting state, a photon
Each kind of atom has a different kind of ladder with a different number of
energy jumps available in different places. As a result, each kind of atom or
ion has a different absorption or emission spectrum. The dark lines of an absorption
spectrum can be laid atop the bright line spectrum of an element and with fit
perfectly. It is therefore possible to take bright line spectra from different
elements and their ionic species and discern what lies within the Sun. From
this work, we know that the Sun has over 70 different elements burning as gas,
including one newly discovered element never seen before on Earth. This elements
unique spectrum was given the name "Helium" since it was first found
in the Sun, and then later on Earth.
Below is the most detailed spectrum of the Sun as imaged from the National
Optical Astronomy Observatory (NOAO) in Chile,
showing as many of the absorption lines as I can imagine are there. What is
so incredible to me is that scientists can sift through such a complex array
of lines and discern which elements and compounds are present. I think the detail
in this image is worth keeping the size of the picture as big as it is.
Okay, so what have we learned here:
Every star makes its own light.
Every star has a different pattern of absorption lines
The absorption lines can tell us what elements are inside a given star
Is it possible that stars can be grouped according to their absorption spectra?
This was the goal of Henry Draper and is the subject
of the next page on the history of the Electromagnetic Spectrum.
Please move forward to HR Diagram
for the next part of this lesson, or you can return to the Sun
Introduction, or to the Syllabus.
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