# Light and Spectra I (Professor Powerpoint)

```The Cosmic
Messenger
Electric and magnetic fields oscillate together, but
perpendicular to each other and the electromagnetic wave
moves in a direction perpendicular to both of the fields.
Light is a electromagnetic wave.
Light
as
a
Wave
The abbreviation used for wavelength is the Greek
letter lambda .
l
• Wavelengths of light are measured in units of
nanometers (nm)
1 nm = 10-9 m
or
&Aring;ngstr&ouml;m (&Aring;):
1 &Aring; = 10-10 m = 0.1 nm
Visible light has wavelengths between
4000 &Aring; and 7000 &Aring; or
400 nm and 700 nm.
Light as a Wave
c = 300,000 km/s = 3*108 m/s
•
•
Light waves are characterized by a
wavelength l and a frequency f.
f and l are related through
f = c/l
Properties of Light
Introduction
Light is radiant energy: it does not
require a medium for travel
Light is an electromagnetic wave
–Light travels at 299,792,458 km/s in a
vacuum (fast enough to circle the Earth
7.5 times in one second)
–However, the speed of light is reduced as it
traverses transparent materials and the speed is
also dependent on color
The Nature of Light
•Early discoveries
–Newton discovered that white light passing through a
prism is comprised of a spectrum of colors.
– Newton
Newton said, “Light is made of very small
particles”
Huygens: “It’s wave-like”
THE VISIBLE ELECTROMAGNETIC SPECTRUM
• Visible light is composed of the colors of the
rainbow.
• Each color is a different wavelength of light.
• Red is at the long wavelength end and violet is at
the short wavelength end.
Photon energy
Note the trends: bluer light has shorter l, higher f, and more
energy. Redder light has longer l , lower f, and less energy.
The electromagnetic spectrum.
4000 A
5000 A
6000 A
7000 A
Which Rays get through our atmosphere?
The Nature of Light
– Light travels at a finite speed, c, not
instantaneously. c  3x108 m / sec
• In 1675 Romer measured the delay in Jupiter’s
moon eclipses
c  3x10 m / sec Or
5
c  3.0x10 km/sec
8
 1.86x10 mi/sec
5
It takes 8 minutes for Sunlight to reach
Earth, and 0.13 seconds for light to go
around the world
Light can also particles as particles,
called photons. A photon has a
specific energy E, proportional to
the frequency f:
E  h 
hc
l
h = 6.626x10-34 Js
h is Planck constant.
The energy of a photon does not depend on
the intensity of the light!!!
Energy depends on its frequency (color)
Light can also behave as a particle and a wave at the
same time. An example of light acting as both a particle
and a wave is the digital camera---the lens refracts
(bends and focuses) waves of light that hit a chargecoupled device (CCD). The photons kick electrons out
of the silicon in the CCD.
Young’s Double slit experiment , showing light acts as waves.
The Doppler Effect (Sound)
–Waves compressed with source moving toward
you; sound pitch is higher.
–Waves are stretched with source moving away
from you; sound pitch is lower.
Doppler effect • similar in light and sound
•Knowing the rest position l0 and the position due
to motion of a spectral line l allows us to calculate
the speed of the object. The size of the shift gives
the speed. Z is the Doppler shift, sometimes called the
red shift.
l  l0
v
l


c
l0
l0
l  l0
z
l0
so,
and
V  cz
Four Ways in Which Light can Interact
with Matter
1. Emission – matter releases energy as
light
2. Absorption – matter takes energy from
light
3. Transmission – matter allows light to
pass through it
4. Reflection – matter repels light in
another direction
Light is a form of energy!
Spectra Lines
There are three laws, known as Kirchhoff's
laws, that govern the spectra and allows us
interpret the spectra we observe.
1. A hot solid, liquid or gas at high pressure
has a continuous spectrum.
Example : the Sun and a light bulb
There is energy at all wavelengths
2. A gas at low pressure and high
temperature will produce emission lines.
A closed tube containing a gas heated to a high temperature
There is energy only at specific
wavelengths.
3. A gas at low pressure in front of a hot
continuum causes absorption lines.
Dark lines appear on the continuum.
As illustrated below, Kirchhoff's laws refer
to three types of spectra: continuum,
emission line, and absorption line.
Low Pressure
Thus when we see a spectrum we can tell
what type of source we are seeing.
Types of spectra: (A) continuous, (B) emission-line, and (C) absorption-line.
Stars have a continuous
and absorption spectrum
Gases around the
Sun absorb photons
Emission Spectrum caused by heating up a gas under low
pressure to a high temperature.
Hydrogen
Helium
Oxygen
Neon
Iron
Astronomers have to find elements among the
spectrum of many elements combined.
4000 A
7000 A
400 nm
700 nm
Red Shift
Compare these spectra.
Spectrum of Hydrogen in Lab
Spectrum a Star
What do these spectra tell us about the star?
Compare these spectra.
Spectrum of Hydrogen in Lab
Spectrum a Star’s
spectral
Hydrogen
What do these spectra tell us about the star?
Compare these spectra.
Hydrogen
Star
What do these spectra tell us about the object?
Compare these spectra.
Spectrum of Hydrogen in Lab
Spectrum a Star…..Day 1
Spectrum a Star…..Day 2
Spectrum a Star…..Day 3
Spectrum a Star…..Day 4
What do these spectra tell us about the star?
ORION
Stars are different colors
due to their temperature.
Composition has no effect
on the color because the
mostly of the same
elements with a small
amount of difference.
Blue white are the hottest
and red stars are the
coolest. The temperature
of a star can be
determined by the highest
point in its trace spectrum.
AA spectrum can be converted to a trace spectrum.
Flux
Hydrogen
Continuum
Absorption Lines
4000
5000
6000
Wavelength
7000
Continuum &amp; Lines
Real stars usually have a blackbody-like continuous
spectrum, upon which absorption lines are superimposed
Spectral Types
• Annie Cannon classified stars according to the
appearance of their spectra.
• The star were classified according to the strength
of the hydrogen absorption lines in the sequence
A, B, C….P.
These spectral classes were changed to a
temperature ordered sequence and some were
This left the following O-B-A-
F-G-K-M.
•Subclasses: …O8, O9, B0, B1, B2…
•Stellar Spectroscopy is the study of the
properties of stars by measuring absorption
line strengths.
•O
B
A
F
G
K
Hottest
M
Coolest
• Oh, Be A Fine Girl (Guy) Kiss Me
A5
O
B
A
F
K7
G
50,000 K
K
M
3,000 K
Sun(G2)
The Spectral Sequence
O
B
Hottest
50,000K
Bluest
A
F
G
K
M
L
Coolest
1300K
Reddest
Spectral Sequence is a Temperature Sequence
O Stars
Hottest Stars: T&gt;30,000 K; Strong
He+ lines; no H lines
B Stars
T = 11,000 - 30,000 K; Strong He
lines; very weak H lines
A Stars
T = 7500 - 11,000 K; Strongest H
lines, Weak Ca+ lines.
F Stars
T = 5900 - 7500 K; H grows weaker
Ca+ grows stronger, weak metals
begin to emerge.
G Stars
T = 5200 - 5900 K; Strong Ca+, Fe+
and other metals dominate,
K Stars
T = 3900 - 5200 K; Strong metal
lines, molecular bands begin to
appear
M Stars
T = 2500 - 3900 K; strong molecular
absorption bands particularly of TiO
```