STUDY GUIDE FOR PART 2 (Astronomy)
Light and Tools
INTRODUCTION:
Ancient Astronomers tried to make geometrical models of the universe that would be able to predict as accurately
as possible the motions of the astronomical objects through the sky. They did not try to explain WHY the motions
occurred, only WHAT those motions would be.
Since Newton formulated his laws of motion and gravitation, people have tried to explain the why of those
astronomical motions in terms of more basic natural laws (like the law of gravity) assuming that the astronomical
objects were composed of material like that of earth and obeyed the same natural laws that apply to the earth.
These attempts have met with tremendous success.
At the end of Part 1 we covered Newton's laws of motion and his law of gravitation. They concerned matter and
motion. But there is yet another basic element of nature that we must try to understand if we are to investigate the
universe under the so very successful assumption that the astronomical objects obey the same natural laws as apply
on earth. This other basic element of nature is LIGHT. Its importance is immediately obvious since light is the
means by which we obtain information about the astronomical objects.
In Section A we investigate the properties of light. Only by knowing the properties of light can we fully extract all
the information the light contains. The light can not only tell us in what direction an astronomical object lies, but also
its surface temperature, the composition of its atmosphere, its speed toward or away from us, and possibly
something of the space that lies between it and us. This will be the major source of the knowledge that will be
covered in the remaining three parts of the course.
In Section B we use our knowledge of the properties of light from Section A to understand the basic tools of the
astronomer. These are the tools that will be used to first gather the light and then to extract the information from the
light. Of course, the basic astronomical tool is the telescope. Both the basic workings of the telescope and the
types of telescopes will be covered. In addition, the spectroscope and the photomultiplier will be briefly described.
In Section C we cover the earth. Having covered the basic ideas of gravity and motion in Part 1 and the idea of
light in Sections A and B of this Part 2, we now consider the earth as a physical body. We need to do this in order to
have a basis for understanding the astronomical objects themselves. Only by comparing and contrasting other
objects to the earth can we really begin to get an idea of what is really out (up) there when we cover Parts 3, 4, and
5.
A. LIGHT AND THE ATOM
[text: Ch. 3, Sections 1-4; Ch. 4, all]
OUTLINE:
1. The speed of light
a) for reference, speed of sound in air is 350 m/s 750 mph
sound moves relative to medium (such as air)
b) speed of light in vacuum = 300,000,000 m/s 670 million mph
light moves relative to what ???
2. The nature of light
a) a form of energy
b) way this energy is transmitted - two theories:
(1) as waves (Electromagnetic oscillations)
(2) as particles (photons)
3. The properties of light
a) color (wavelength or frequency of waves; energy of photon)
b) reflection (both waves and photons [particles] reflect)
c) refraction (bending: waves predict correct behavior, photons don't)
d) diffraction (shadows: waves spread out, photons don't)
e) electricity theory (light as electromagnetic waves)
f) energy theory (photon emission correct, waves fail)
