Chapter 3 - Radiation With Doctor Bones aka Don R. Mueller, Ph.D.

1. Astonishing Astronomy 101
With Doctor Bones (Don R. Mueller, Ph.D.)
Educator
Entertainer
J
U
G
G
L
E
R
Scientist
Science
Explorer

2. Chapter 3 Radiation

3. • It is far too difficult to visit stars and most planets, so how can we obtain
useful information about them? We can examine the types of EM
radiation that they produce.
• The colors we see define the visible spectrum ROYGBIV, but
there is much more to light than this very narrow band of color!

4. Wavelength
The colors we see are determined by
the wavelength of light.
Wavelength is the distance between
successive crests (or troughs) in
the wave.
The wavelength is given by the
Greek letter  (lambda) and the
frequency, by the letter ν(nu).
• Wavelengths of visible light are very small!
– Deep red light has a wavelength of 710-7
meters or 700 nanometers (nm).
– Violet light has a wavelength of 410-7
meters or 400 nm.

5. Frequency
• Sometimes it is more convenient
to talk about light in terms of
the frequency: how fast
successive crests pass by a given
point (i.e., cycles/time).
• You can think of frequency as a
measure of how fast you bob up
and down as the waves pass by.
• Frequency has units of Hertz:
1Hz = 1/s
• Longer wavelengths have lower
frequencies than do shorter
wavelengths, which have higher
frequencies.
• Frequency and wavelength are related by:
c
‘c’ (Latin: celera or “swift”) is the speed of light.

6. Frequency versus wavelength
• The speed of light is a constant. This means that as the wavelength of
electromagnetic radiation decreases, its frequency must increase. This is
illustrated by the figure below, which compares wave (a) to wave (b). Wave (a)
is of longer wavelength than wave (b), which means that the frequency (wave
cycles per second) of wave (a) must be less than the frequency of wave (b). By
comparing the number of peaks for the two waves it is easy to see that wave (b)
is the higher frequency wave. Check it out!

7. White Light
• Light from the Sun arrives
with mixed wavelengths.
We perceive this mixture of
colors as white light.
• Newton demonstrated that
white light could be
dispersed (split) into its
component colors using a
prism and then recombined
back into white light using
a lens.

8. The Electromagnetic Spectrum
In order of increasing photon energy: E = hν = hc/
( Planck’s constant h = 6.626  10-34 J s )
 Radio waves are very long wavelength photons.
 Microwaves are at the upper end of the radio portion of the spectrum.
 Infrared waves are just longer in wavelength than the visible.
 Ultraviolet waves are just shorter in wavelength than visible light.
 X-rays come mostly from stellar sources and can penetrate materials
such as skin, muscle and bone.
 Gamma rays are of very short wavelength and high energy.
Side Notes: The designations, Near and Far, pertain to “near the visible”
and “far from the visible,” respectively.

9. Pioneers of Electromagnetic Radiation
The Hertz is named in honor of Heinrich Hertz (1857-1894), who was the first to
demonstrate the existence of electromagnetic radiation. His experimental work
in this area confirmed the theoretical predictions of James Clerk Maxwell
(1831-1879) who proposed that light was a form of electromagnetic radiation.
Maxwell's equations, describe the behavior of electric and magnetic fields and
how these fields interact with matter.
H. Hertz J. Clerk Maxwell

10. EM Spectrum
EM Spectrum Song
http://www.youtube.com/watch?v=bjOGNVH3D4Y&feature=related

11. Measuring Temperature
• Temperature is a
measure of the random
motion of atoms and
molecules in substances.
• Low temperatures have
atoms moving slowly.
• High temperatures have
atoms moving rapidly.
• The Kelvin temperature
scale was designed to
reflect this. Even at
absolute zero (0  K)
when atoms are not
moving translationally,
they are still vibrating.

12. Thermometry

13. Temperature
Conversions:
Celsius (centigrade)
temperature formulae:
Celsius to Fahrenheit:
[°F] = 9/5[°C] + 32
[°C] = 5/9([°F] − 32)
Celsius to Kelvin
[K] = [°C] + 273.15
[°C] = [K] − 273.15

14. Thermometer Links
• How a thermometer works
http://www.youtube.com/watch?v=ExJjXVhqM4I
• Galileo thermometer
http://www.youtube.com/watch?v=917UC2MZOGU
• How the Galileo thermometer works
http://www.howstuffworks.com/question663.htm
• Microwaving A Galileo Thermometer
http://www.youtube.com/watch?v=kLrQa76ATl0

15. Heat: A measure of random thermal motion
The natural direction of heat flow between two objects depends on
temperature. Therefore, heat flows from hotter to cooler.
Connecting Heat (Q) to Temperature (T): Q = mCΔT
Calculate the amount of heat (Q) used to raise the temperature of a
mass (m) = 25.0 kg of copper (Cu) from an initial temperature
Ti = 28.0˚C to a final temperature Tf = 315.0˚C.
Q = mCuCCu ΔT CCu = 0.093 cal/(g ∙°C). 1 kg = 1000 g
Q = (25.0 kg)(1000 g/kg)[0.093 cal/(g ∙°C)](315.0˚C - 28.0˚C)
= 667275 cal (667000 cal) or 6.67 x 105 cal or 6.67 x 102 kcal

16. Heating diagram for water
Positions on the temperature versus energy diagram
(A-G) for water
E
100.0 G
(°C)
F D
0.0
C
A B
Energy (Joules)
___A___ Ice
___B___ Ice + Water
___C___ Water
___D__ _Water + Steam
___E____ Steam
___F____ ΔHfusion
___G____ ΔHvaporization
P = 1.0 atm
Tf = 0.0 °C
Tb = 100.0 °C

17. The Sun
as a
Furnace
• A blackbody is a theoretical object that
absorbs 100% of the radiation that
strikes it. It reflects no radiation and
therefore appears perfectly black.
• Stars are near black-body radiators.
Most of the light directed at a star is
absorbed. A star is capable of absorbing
and emitting all wavelengths of
electromagnetic radiation. Although
most blackbodies are solids, the gas
particles in stars are so dense that the
star functions as a blackbody, absorbing
the majority of incident radiant energy.

18. Blackbody Cavity
• A simple example of a blackbody
radiator is the furnace. If there is a
small hole in the furnace, heat energy
can enter from outside. This energy is
absorbed by the walls, which become
hot, emitting thermal radiation. The
radiation may be absorbed by another
part of the furnace or exit through the
small hole. The escaping radiation
may contain any wavelength. The
furnace is said to be in equilibrium
when the rates for absorption and
emission are equal.
• The frequency ν of radiation is given
by ν = c/λ.
• High energy means short wavelengths
and high frequencies: E = hν = hc/λ
There are many reflections and absorptions.
Photons enter, but very few get out. The cavity
features radiation, which is homogeneous and
isotropic.

19. The Blackbody Spectrum
• As a blackbody is heated,
its atoms move faster and
faster.
• Atoms emit photons with
energies proportional to the
collision energy.
• Atoms colliding lightly, produce
long-wavelength radiation.
• Atoms colliding very hard
produce short-wavelength
radiation.
• As the body gets hotter, both the
number of collisions and the
number of hard collisions
increases.
• Stefan-Boltzmann Law:
Brightness rises rapidly with
temperature.
Gentle collisions
Hard collisions

20. Blackbody radiation curves
http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html
• The blackbody radiation curve
shows that the blackbody does
radiate energy at every wavelength
as the curve gets infinitely close to
the wavelength axis, but the curve
never reaches it. It also shows that
the blackbody emits at a peak
wavelength, whereby most of the
radiant energy is emitted.
• As the temperature increases, the
peak wavelength emitted by the
blackbody decreases in wavelength.
• As temperature increases, the total
energy emitted increases, because
the total area under the curve
increases.

21. Results of Increased Collisions
• Higher collision frequencies
mean that more photons are
being emitted: hotter objects
are brighter than cooler objects.
• As higher energy photons are
being emitted, objects appear to
shift in color from Red to
Orange to Yellow and so on;
shifting to lower wavelengths
(higher frequencies).
• Wien’s Law describes this.

22. Wien’s Law: Hotter bodies emit more strongly at shorter wavelengths
(Measuring the Temperatures of Astronomical Objects)
Wien’s Law:
To estimate the temperatures
T(K) of stars.
• We just need to measure the
wavelength (max) at which
the star emits the most
photons
• Solving for T:
If the wavelength of maximum emission
(max) for the spectral distribution of
the blackbody is plotted versus 1/T, a
straight line is obtained.
max
6
nmK109.2


T

23. Stefan-Boltzmann Law:
The luminosity of a hot body rises rapidly with temperature
The total emitted radiate energy is
proportional to the 4th power of
the temperature T:
• If we know an object’s
temperature (T), we can
calculate how much energy
the object is emitting using
the SB law:
• The power P is in Watts, area
A in square meters and the
Stefan-Boltzmann constant:
 = 5.6710-8 Watts/m2K4
4
σTP/A 

24. Electromagnetic radiation
• Electromagnetic radiation is emitted by all objects
No matter how “dim” they might be.

25. The Doppler Shift (Effect)
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

26. Doppler Shift in Sound
http://en.wikipedia.org/wiki/Doppler_effect
Squeezing Waves
• You have probably experienced the
Doppler shift (effect) in sound.
• Standing on the sidewalk, watching
cars whiz by.
• As a car approaches, the sound from
the car has a higher pitch
(frequency) that is, moving to shorter
wavelengths.
– As the car passes by, the
sound shifts over to longer
wavelengths, a lower pitch
(frequency)
– Police radar guns work on the
same principle. The waves
reflected off the car will be
shifted by an amount that
corresponds to the car’s speed.

27. Doppler Shift in Light
• If an object is emitting light
and is moving directly
toward you, the light you see
will be shifted to slightly
shorter wavelengths –
toward the blue end of the
spectrum:
Blue-shifted
• Likewise, if the object is
moving away from you, the
light will be Red-shifted.
cVR



If we detect a wavelength shift of  away from
the expected wavelength , the radial velocity VR
of the object is:

28. Tony Soprano on the “Sonic Boom”
Ba Da Boom!
Ba Da
Bing!

29. Tony says:
One reason that we observe
broadened absorption and emission
lines is because collisions between
atoms produce Doppler shifts.
Listen to the Professor or I’ll be back…