Transcript light
Astronomy 1020-H
Stellar Astronomy
Spring_2016
Day-12
Course Announcements
•
•
1st Quarter observing night: TONIGHT @ 7:30pm
READ through the “Format for the Report” BEFORE
you come.
N
5
Mean
73.3
Std. Dev.
14.2
Median
79.2
Mode
---
Min.
56
Max.
87.2
Curve
---
The Origin and Nature of Light
Lab This Week
•
The Spectrometer
•
What you need to know:
You get to visualize the spectra from various
sources.
Reading ahead in Chapter 5 will help.
•
•
Lab This Week
The Spectrometer
Almost all
knowledge of the
universe beyond
Earth comes from
light.
Light can tell us
about objects in
space: temperature,
composition,
speeds, and more.
What Light Can Tell Us.
Photometry – Study of Brightness:
Luminosity, Est. of Distance, Rough Temp. of Star
Spectroscopy – Study of the EM Spectrum:
Composition, Radial Velocity, Temp., Surface-gravity
Mass for compact objects
Astrometry – Study of Positions:
Tangential Motion, Distance
Polarimetry – Study of Polarization of Light:
Magnetic Field
Spectrum of the Sun
But, what is light?
Newton, Remember Newton? He did more than just
calculus and mechanics.
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
particle and a wave depending on how you observe it.
Particle Nature
Thomas Young’s interference experiment
Wave Nature
But, where does light actually
come from?
Accelerating charges (think electrons
and protons) produce light –
electromagnetic radiation!
Photons
Light also behaves as a particle.
Photon: particle of light.
Photons carry energy and can have
different amounts of energy.
Photons with high energy = light with high
frequency.
Photons with low energy = light with low
frequency.
Atoms can absorb or emit photons.
Einstein (Nobel Prize)
In 1905 Einstein
calculated the energy of
a particle of light
(photon) and proposed
the photoelectric effect.
photon
Ephoton = hc/ = hn
e-
Light moves at 300,000 km/s in a vacuum.
First measured by Rømer when observing
Jupiter’s moons.
Speed is slower in other materials, e.g.,
glass.
Wavelength ():
length between
crests.
Amplitude: height.
Frequency (f):
number of waves
that pass by each
second.
Period (P): time to
complete one cycle.
A long wavelength
means low
frequency.
A short wavelength
means high
frequency.
The speed of light, c,
is constant.
S
p
e
e
d
c
W
a
v
e
l
e
n
g
t
h
o
r
F
r
e
q
u
e
n
c
y
f
Light is a wave of combined electricity and
magnetism, called an electromagnetic wave.
Changing electric and magnetic fields create a
self-sustaining electromagnetic wave.
Maxwell – EM Field Theory
Scottish physicist James Clerk
Maxwell showed mathematically
in the 1860s that light must be a
combination of electric and
magnetic fields.
MATH TOOLS 5.1
Knowing the speed of light and one other
variable, either the wavelength or frequency
of the light in question, you can find the
remaining quantity.
Example: Find the wavelength of the light
wave coming from a radio station
broadcasting on 770 AM:
EM Spectrum
Like the flavors
of Ice cream –
they each
provide us with
different
information.
But what do
you get when
you put all the
flavors (light)
together?