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seasons,
and telescopic aspects of the planets; now these
are only primary matters. Once it considered
stars as mere fixed points of light; now it
studies them as suns, determines their age,
size, color, movements, chemical constitution,
and the revolution of their planets. Once it
considered space as empty; now it knows that
every cubic inch of it quivers with greater
intensity of force than that which is visible
in Niagara. Every inch of surface that can be
conceived of between suns is more wave-tossed
than the ocean in a storm.
The invention of the telescope constituted one
era in Astronomy; its
perfection in our day, another; and the discoveries
of the spectroscope
a third--no less important than either of the
others. New discoveries
are made every day with the advancement of telescopes.
The Hubble
space telescope has let man see further into
the universe then ever
before. Astronomy and space science is an ever
changing study, and
possibly the most exciting of the sciences.
It is for one reason that this
book was written, to hopefully interest more
people in the exciting
study of the universe around us.
Why
Study Light?
For most of history, humans have used visible
light to explore the
skies. With basic tools and the human eye, we
developed sophisticated
methods of time keeping and calendars. Telescopes
were invented in
the 17th century. Astronomers then mapped the
sky in greater
detailstill with visible light. They learned
about the temperature,
constituents, distribution, and the motions
of stars.
There are two main techniques for analyzing
starlight. One is called
spectroscopy and the other photometry. Spectroscopy
spreads out the
different wavelengths of light into a spectrum
for study. Photometry
measures the quantity of light in specific wavelengths
or by combining
all wavelengths. Astronomers use many filters
in their work. Filters
help astronomers analyze particular components
of the spectrum. For
example, a red filter blocks out all visible
light wavelengths except
those that fall around 600 nanometers (it lets
through red light).
Introduction
to Light
Light is a form of radiant energy or energy
that travels in waves. Since
Greek times, scientists have debated the nature
of light. Physicists now
recognize that light sometimes behaves like
waves and, at other times,
like particles. When moving from place to place,
light acts like a
system of waves. In empty space, light has a
fixed speed and the
wavelength can be measured. In the past 300
years, scientists have
improved the way they measure the speed of light,
and they have
determined that it travels at nearly 299,792
kilometers, or 186,281
miles, per second.
When we talk about light, we usually mean any
radiation that we can
see. These wavelengths range from about 16/1,000,000
of an inch
to 32/1,000,000 of an inch. There are other
kinds of radiation such as
ultraviolet light and infrared light, but their
wavelengths are shorter
or longer than the visible light wavelengths.
When light hits some form
of matter, it behaves in different ways. When
it strikes an opaque
object, it makes a shadow, but light does bend
around obstacles. The
bending of light around edges or around small
slits is called diffraction
and makes patterns of bands or fringes.
All light can be traced to certain energy sources,
like the Sun, an
electric bulb, or a match, but most of what
hits the eye is reflected
light. When light strikes some materials, it
is bounced off or reflected.
If the material is not opaque, the light goes
through it at a slower
speed, and it is bent or refracted. Some light
is absorbed into the
material and changed into other forms of energy,
usually heat energy.
The light waves make the electrons in the materials
vibrate and this
kinetic energy or movement energy makes heat.
Friction of the moving
electrons makes heat.
Experiments With Light
A light set in a room is seen from every place;
hence light streams in
every possible direction. If put in the centre
of a hollow sphere, every
point of the surface will be equally illumined.
If put in a sphere of twice
the diameter, the same light will fall on all
the larger surface. The
surfaces of spheres are as the squares of their
diameters; hence, in
the larger sphere the surface is illumined only
one-quarter as much as
the smaller. The same is true of large and small
rooms. In Fig. 7 it is
apparent that the light that falls on the first
square is spread, at twice
the distance, over the second square, which
is four times as large, and
at three times the distance over nine times
the surface. The varying amount of light received
by each planet is also shown in fractions
above each world, the amount received by the
earth being 1.
Fig.
7.
Fig. 8.--Measuring Intensities of Light.
The intensity of light is easily measured. Let
two lights of different
brightness, as in Fig. 8, cast shadows on the
same screen. Arrange
them as to distance so that both shadows shall
be equally dark. Let
them fall side by side, and study them carefully.
Measure
the respective distances. Suppose one is twenty
inches, the other forty. Light varies as the
square of the distance: the square of 20 is
400, of
40 is 1600. Divide 1600 by 400, and the result
is that one light is four
times as bright as the other.
Fig.
9.--Reflection and Diffusion of Light.
Light can be handled, directed, and bent, as
well as iron bars. Darken
a room and admit a beam of sunlight through
a shutter, or a ray of
lamp-light through the key-hole. If there is
dust in the room it will be
observed that light goes in straight lines.
Because of this men are able
to arrange houses and trees in rows, the hunter
aims his rifle
correctly, and the astronomer projects straight
lines to infinity. Take a
hand-mirror, or better, a piece of glass coated
on one side with black
varnish, and you can send your ray anywhere.
By using two mirrors,
or having an assistant and using several, you
can cause a ray of light
to turn as many corners as you please.
Set a small light near one edge of a mirror;
then, by putting the eye
near the opposite edge, you see almost as many
flames as you please
from the multiplied reflections. How can this
be accounted for?
Into your beam of sunlight, admitted through
a half-inch hole, put the
mirror at an oblique angle; you can arrange
it so as to throw half a
dozen bright spots on the opposite wall.
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Be a Stargazer