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Chapter

Chapter Three Notes: Radiation


Department
Astronomy
Course Code
ASTA01H3
Professor
Brian Wilson

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Radiation: Information from the
Cosmos
3.1 Information from the skies
-The Andromeda galaxy is comparable in diameter to the full moon
-lies roughly 2.5 million light years away
-even if a space probe could travel at the speed of light, it would need 2.5
million years to reach this galaxy and 2.5 million more to return
Since it is practically impossible to travel such far distances in the
universe, how do astronomers know anything about objects far from
earth?
We use the laws of physics as we know them here on earth to interpret the
electromagnetic radiation emitted by those objects
Light and radiation
-Radiation: any way in which energy transmitted through space from one
point to another without the need for any physical connection b/w the two
locations.
-The term electromagnetic just means that the energy carried is in the
form of rapidly fluctuating electric and magnetic fields.
-Electromagnetic theory provides us with answers to questions regarding
masses, temperature and proportion of galaxies and planets in the
universe.
-Visible light is the particular type of electromagnetic radiation to which
our human eyes happen to be sensitive.
-Modern instruments can detect many forms of invisible electromagnetic
radiation which goes completely unnoticed by our eyes such as radio,
infrared and ultraviolet waves, also x rays and gamma rays
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-Despite the different names, the words light, rays, radiation and waves all
really refer to the same thing.
-The names are just historical accidents because it took many years for
scientists to realize that all these forms of radiation are in reality one and
the same physical phenomenon.
Wave Motion
-All electromagnetic radiation travel through space in the form of waves
-A wave is a way in which energy is transferred from place to place without
the physical movement of material from one location to another.
-In wave motion the energy is carried by a disturbance of some sort
-This disturbance, wtv its nature, occurs in a distinctive repeating pattern
-Similar to sound waves in the air, or ripples on the surface of a pond
-A wave is not a physical object
-An example of a disturbance is throwing a pebble into a lake; the ripples
then might cause a nearby twig to float somewhere else.
-(refer to figure 3.3 on pg 59) ; shows how wave properties are quantified
and illustrates terminology
-The waves period is the number of seconds needed for the wave to repeat
itself at any given point in space
-The wavelength is the number of meters needed for the wave to repeat
itself at a given moment in time
-It can be measured as the distance between two adjacent wave crests, two
adjacent wave troughs, or any two similar points on adjacent wave cycles.
-A wave moves a distance equal to 1 wavelength in one period
-The max departure of the wave from the undisturbed state is called the
waves amplitude. (eg. Flat pond surface)
-The number of wave crests passing any given point per unit time is called
the waves frequency
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-If a wave of a given wavelength moves at high speed, then many crests
pass per second and the frequency is high.
-Conversely, if the same wave moves slowly, then its frequency will be low.
-The frequency of a wave is just the reciprocal of the waves period :
Frequency = 1/period (the slash means to divide ;)
-Frequency is expressed in units of inverse time that is; 1/second, or
cycles per second, called hertz (Hz)
-Thus a wave with a period of 5 seconds (5 s) has a frequency of
(1/5) cycle/s = 0.2Hz, meaning that one wave crest passes a given
point in space every 5 seconds
-b/c a wave travels one wavelength in one period, it follows that the
wave velocity is simply equal to the wavelength divided by the
period :
Velocity = wavelength/period
-since the period is the reciprocal of the frequency we can
equivalently write this relationship as
Velocity = wavelength x frequency
-Velocity represents the speed of light.
-Notice that wavelength and wave frequency are inversely
related-doubling one halves the other.
The Components of Visible Light
-White light is a spectrum of colours which we
divide into six major hues; red, orange, yellow,
green, blue, and violet.
-Isaac Newton was the one who performed the
white light through the prism experiment
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