The Nature of Light Or, Illuminating the Use of Light in Astronomical Studies

1
The Nature of LightOr, Illuminating the Use of
Light in Astronomical Studies

• Fact Visible light is a small portion of the
  electromagnetic spectrum.
• Application Astronomers examine the entire
  electromagnetic spectrum, not just visible light.
• Fact Each element has its own unique set of
  spectral lines.
• Application Astronomers can determine the
  composition of light sources by examining the
  spectral lines of the light from the sources.

2
Redefining a Wave
Wave A way in which energy and information may
be transferred from place to place without
physical movement of material from one location
to another. The energy is carried by a
disturbance of some sort that moves in a
distinctive, repeating pattern. A wave is
characterized by its velocity, frequency, and
wavelength.
Image is from page 45 and the definition is from
the glossary of your lecture textbook, Astronomy
A Beginners Guide to the Universe
How does an ocean wave caused by the tide not fit
this definition of wave?
3
Waves and Their Medium
The material medium of a wave is the stuff
through which the waves travel. Ripples on a pond
are the up and down motion of water. Sound is the
compression and rarefaction of air.
Electromagnetic waves do not have a material
medium. (This will become our starting point for
special relativity.) Instead we think of the
electromagnetic waves as being fluctuations in
the electromagnetic field. The electromagnetic
field does not have properties independent of
electromagnetic waves.
From page 47 of the lecture textbook Astronomy
A Beginners Guide to the Universe
4
Properties of a Wave
General Properties of Waves
Wave Period The time required for a wave to
repeat itself at a specific point in
space. Wavelength Distance from one wave crest
to the next at any instant in time. Frequency
The number of wave crests passing any given point
per unit time. Wave Velocity The speed with
which a wave moves.
These definitions are from the glossary of your
lecture textbook, Astronomy A Beginners Guide
to the Universe
1
Frequency
Wave Velocity Wavelength x Frequency
Wave Period
From page 46 of the lecture textbook Astronomy
A Beginners Guide to the Universe
5
Light as a Wave
Electromagnetic waves can be of different amounts
of energy. The amount of energy that a given
wave has ends up defining how it interacts with
matter. As such when we classify electromagnetic
waves by how they interact with matter, we are
really dividing it up into different ranges of
energy. For example some electromagnetic waves
have just the right energy that they just sort of
bounce atoms around. Other electromagnetic waves
have enough energy to break the covalent bonds of
our DNA. Our eyes are capable of detecting a
certain range of energy. The electromagnetic
waves of this small range of energies are called
visible light.
6
Light as a Wave
Electromagnetic energy is directly proportional
to the frequency of the wave. Electromagnetic
Energy Planks Constant Frequency Light has
one set speed in a vacuum speed of light c
3.0 x 108 m/s Recall Velocity Wavelength x
Frequency. So each energy corresponds to a
certain wavelength and frequency of the
electromagnetic wave.
From page 49 of the lecture textbook Astronomy
A Beginners Guide to the Universe
7
(No Transcript)
8
(No Transcript)