Download What is light?

The Origin and Nature of
Lecture 9
(Part I: Light and Luminosity)
•What is LIGHT?
•How is the Luminosity of a star related to it’s Temperature & Size
[Stephan –Boltzman Law]
What is light?
• In the 17th Century, Isaac Newton argued
that light was composed of little particles
while Christian Huygens suggested that light
travels in the form of waves.
• In the 19th and 20th Century Maxwell,
Young, Einstein and others were able to
show that light behaves both like a wave
and a particle depending on how you
observe it.
Thomas Young’s interference experiment
(1801)
Confirmed the wave nature of light.
Scottish physicist James Clerk Maxwell showed
mathematically in the 1860s that light must be a
combination of electric and magnetic fields.
In 1905 Einstein calculated the energy of
a particle of light (a photon) and
proposed the photoelectric effect.
(Confirmed the particle nature of light.)
Ephoton = hc/l or Ephoton = hf
Which light is higher energy:
A) Radio waves
photon
e-
B) Ultraviolet radiation
C) Yellow light
D) X-rays
But, where does light actually
come from?
Light comes from
the acceleration of
charged particles
(such as electrons
and protons)
But, where does light actually
come from?
electron
Accelerating charges produce
light – electromagnetic radiation!
What if all colors are
combined?
Speed of light =
wavelength x
frequency
c=lf
Speed of light is
CONSTANT in a
vacuum (space has
Luminosity is the total energy (light)
emitted by an object in each
second.
Like Watts of a light bulb.
Stefan-Boltzmann law
Luminosity depends on a surface area (A), and its
temperature (T4)
Luminosity = 5.67x10-8(A)T4
Big and
? objects have greater luminosity than __?__ Cool
objects
Think hot plates or burners on an electric stove!
Luminosity = 5.67x10-8(A)T4
The stars Antares and Mimosa each have the
same luminosity. Antares is cooler than
Mimosa.
Which star is larger?
a) Antares
b) Mimosa
c) the same size
d) not enough information
Luminosity = 5.67x10-8(A)T4
Rigel is much more luminous than Sirius B.
Rigel and Sirius B have the same
temperature.
Which star has the greater surface area?
a) Rigel
b) Sirius B
c) the same
2) You observe two stars with the same luminosity and
determine that one is larger than the other.
Which star has the greater temperature?
a) the smaller star
b) the larger star
c) The temperatures are the same.
You observe a very large and very hot star in the
constellation Orion. On the same night,
you observe another star in Orion that is much
smaller but has the same temperature.
Which star is more luminous?
a) the larger star
b) the smaller star
c) They have the same temperature.
d) There is insufficient information to determine this.
Which star is Hot and Dim?
Below we are building a “Hertzsprung–Russell Diagram”…
10,000
A
D
1,000
100
10
C
1
.1
.01
B
E
.001
.0001
20,000
10,000
Temperature (K)
5,000
Which star is Cool and Dim?
10,000
A
D
1,000
100
10
C
1
.1
.01
B
E
.001
Temperature (K)
.0001
20,000
10,000
Temperature (K)
5,000
Which star is Largest?
10,000
A
D
1,000
100
10
C
1
.1
.01
B
E
.001
Temperature (K)
.0001
20,000
10,000
Temperature (K)
5,000
Which star is smallest?
10,000
A
D
1,000
100
10
C
1
.1
.01
B
E
.001
Temperature (K)
.0001
20,000
10,000
Temperature (K)
5,000