Properties of Light
Welcome to the first page in this unit which will attempt to familiarize you
with the properties of light, and the value of these properties to the astronomer.
You may walk outside on any given sunny afternoon are notice the bright light
from the sun. You may see your shadow behind you. You may notice the blue color
of the sky. You certainly will notice how painful it becomes to look directly
at the sun (of course this is not advised for anyone since sunlight is sufficient
to cause permanent damage to your retina). When you go outside after sunset,
the sky looks black, and you see little white dots of light speckling the sky
overhead. Where does all this light come from, and what does it mean?
Light holds the clues to the workings of stars, and all stars manufacture
light. While you will learn more of this interesting phenomenon later, accept
for now that all stars make light. But what is light? Isaac Newton was the first
person to really study light. When he let light pass through a glass prism,
he discovered that white light is actually a combination of all the colors.
The absence of these colors results in black, and the combination of all of
these colors results in white. While you may beg to differ at this time based
on your experience with crayons, remember that we are talking about colored
light more than the mixing of paints or crayon scribblings.
In the image
to your right, you will see a very simple drawing of how this works. Sunlight
passes through a glass prism and the colors show up against a white background.
The combination of all the colors is white. This is how a rainbow works after
a rain. You are living in Minnesota where the storms typically move from west
to east, so a great time to see rainbows is after the storm has passed and during
the afternoon. The clouds will drop rain toward your east direction, while the
sun is peeking through the clouds to your west. Sunlight strikes the water droplets,
scattering the white light into its constituent color lines, and voila! a nice
rainbow. Oh, I am so sorry to have ruined the almost magical moment for you.
If you want, you may still watch the rainbow in rapt awe, but there is no real
pot of gold at the end of the rainbow!
excellent image shows a greater detail of the spectrum of light, with closer
attention paid to the many colors present and the blending of the colors from
red to violet. The colors follow the acronym ROYGBIV ... red, orange, yellow,
green, blue, indigo, violet. But ... there is more to light than meets the eye.
image shows all of what we call "light." The diagram is of the electromagnetic
spectrum. Starlight, including that which comes from the sun, is really much
more than the visible part which we see. Light is made up of all sorts of different
forms, but one thing is common to all. All forms of starlight travel in waves,
and all waves travel at a speed of 300,000 km/sec. The difference between the
forms is the energy of the wave, described in this diagram as the frequency
of the wave, or better yet, the actual distance between crests of the waves
... called "wavelengths." In the beginning of the course, you were
introduced to units of length measurement called nanometers (nm). These are
billionths of a meter. The portion of the entire spectrum of starlight which
are eyes are formatted to notice is between 400 and 700 nm in wavelength. Higher
energy starlight will have shorter wavelengths ... ultraviolet at 300-100 nm,
x-rays at 1 nm, and extremely dangerous gamma rays which are hundreds to thousands
of times shorter than a nanometer.
Longer wavelengths are also generated in stars, such as infrared which can
be measured in 1000's of nanometers, miocrowaves like your home over near a
mm in wavelength, and even radio waves similar to what you tune into while driving
a car, and these can be many km in length. ALL FORMS OF LIGHT ARE EMITTED FROM
The next important lesson to learn about this electromagnetic spectrum is
found in the danger of various forms of this light. We call the spectrum to
your left a chart of electromagnetic radiation, since light generated in stars
radiates out in wave form. X-rays and Gamma rays are deadly forms of radiation.
Hospitals are careful to track the total number of x-rays received in your lifetime.
This radiation causes damage to cellular DNA, and especially in those cells
which divide rapidly (skin cells, liver cells, and sex cells). Cancer often
results from permanent damage to any of these cells following short exposures
to x-rays. Worse are gamma rays. They are lethal, and most victims of an atomic
blast suffer radiation burns and eventual death. Only Dr. Bruce Banner has ever
survived massive doses of gamma radiation, but his skin occasionally turns green
and he gets really muscular and big. Then he starts wrecking entire towns in
his raging form as the "Incredible Hulk." Even ultraviolet radiation
is dangerous, and for the same reasons as x-rays and gamma rays. This form of
light mutates cells and causes skin cancer unless you are genetically endowed
with great tanning ability or wear sunscreen. Fortunately, various naturally
occurring gases in our Earth atmosphere block all gamma and x-rays, as well
as most of the ultraviolet. Fear of chlorofluorocarbons degrading the ozone
layer and allowing more UV light to penetrate are real, but the ozone hole over
Antarctica has been demonstrated to be nonexistent recently. Still, we must
take caution. One molecule of CFC will stay up in the upper atmosphere for years
and may destroy hundreds of thousands of ozone molecules. Yes, lightning can
restore zone, but not as fast as a CFC destroys it.
Infrared radiation is not that bad, and actually is more measured as heat
than actual light. If you had an infrared camera, you would see an entirely
different world. We would perceive heat signatures instead of visible images
to our eyes. These cameras are useful because IR penetrates some substances
readily. Firefighters use an IR camera to search for people in a smoky room,
since IR penetrates smoke and we can see warm bodies. Police use an IR camera
when chasing bad guys in the woods at night. And army intelligence soldiers
use an IR camera to see right through brick walls.
So you see, light is really pretty interesting, and much more than meets your
eye. With this basic information in your minds, please
move forward to Starlight for a more detailed
explanation of the properties of light as they relate to stars and how we have
come to learn more about stars than simple pretty dots in the sky for our viewing
pleasure or wishlists ... or just return to the Introduction
to Light and Telescope, the Syllabus,
or the Home page.
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