Download Understanding Light - Indiana University Astronomy

H205
Cosmic Origins
APOD
Syllabus
Oncourse
Light in Astronomy (Ch. 5)
EP1 Due Wednesday
Applet: RGB
What does “electromagnetic” mean?
Wavelength
and Frequency
Properties of waves
speed (distance per second)
wavelength (length)
frequency (cycles per second)
speed = wavelength x frequency
speed = wavelength x frequency
Examples – A Sound Wave
Speed – about 340 meters per second
Frequency – say, middle C (256-278
cycles per second depending on the scale)
What is the wavelength?
340 m/sec = W x 278 per second
340(m / sec)
W
 1.2(meters)
262(/ sec)
speed = wavelength x frequency
Example – A Tsunami Wave
speed – about 700 km/hour
(200 m/sec)
Wavelength – about 50 km
What is the frequency of the
wave?
700 (km/hour) = 50 (km) x frequency
frequency = 700/50 = 15 cycles per hour
speed = wavelength x frequency
Examples – An Electromagnetic Wave
 speed – 300,000 km per second (3 x 108 meters
per second)
 frequency – say, one billion cycles per second
(109 cycles per second)
 What is the wavelength?
 What kind of light is this?
3 x 108 m/sec = W x 109 /sec
3x108 (m / sec)
W (meters) 
 0.3(meters)
9
10 (/ sec)
X-Rays and
Gamma Rays
 X-rays and gamma rays absorbed in the Earth’s
atmosphere
 Observatories must be sent into space
 Produced by matter heated to millions of degrees
 Caused by cosmic explosions, high speed collisions
 Led to the discovery of black holes in space
 Allow us to study the hottest regions of the Sun's
atmosphere
Ultra-Violet
 Ultraviolet light is blocked by the ozone layer
 Astronomers use balloons, rockets, and satellites
 Most stars are too cool to emit much UV light
 UV sources include very young stars, some very
old stars, white dwarfs stars, active galaxies and
quasars
 Discoveries include a hot gaseous halo surrounding
our own galaxy that glows in the UV
Visible
Light
• Ground-based observatories “see” during clear sky
evenings
• Adaptive optics eliminate the blurring effects of the
atmosphere
• In space we can get a much clearer view of the cosmos
• Visible light observations give us the most detailed
views of our solar system, and have brought us
fantastic images of nebulae and galaxies
• Stars are brightest in visible light
Infrared
 Some bands of infrared light can be observed by ground-based
observatories
 To view the rest of the infrared universe we need to use space
based observatories or high-flying aircraft
 Infrared is primarily heat radiation
 Infrared sees through thick regions of dust in space to peer
into star-forming regions and into the central areas of our
galaxy
 Cool stars and cold interstellar clouds which are invisible in
optical light are also observed in the infrared
• Wavelengths of about 1 mm to 1 m
• The cosmic microwave background (electromagnetic
radiation left over from the Big Bang) is now
observed in the microwave part of the spectrum
• Cold interstellar clouds are microwave sources
• The early stages of star formation are observed in
microwaves
Radio
• Radio waves are long compared to other EM waves
• Most radio radiation reaches the ground and can be
detected during the day as well as during the night
• Radio telescopes use a large metal dish to collect
radio waves
• Radio telescopes first detected the radiation left
over from the Big Bang
• Supernovae, quasars, pulsars, regions of gas
between the stars, and interstellar molecules
radiate in the radio
Multiwavelength
Orion
Optical
X-Ray
Infrared
UV
Radio
Exploring Color
Astronomy Rules!
Astronomy
is looking up!
Where does light come from?
THERMAL
EMISSION
ATOMIC
EMISSION
Thermal radiators emit light at
all wavelengths
Atomic emission occurs only at
particular wavelengths
Cool matter glows
primarily with radio
or infrared light
All matter glows
with light
Warmer matter glows
with higher energy
light
Even hotter matter
glows blue hot
Matter at about
10,000 degrees centigrad
glows white hot
The glow of matter because of its
temperature is called
Thermal radiators emit light at all wavelengths
Cooler object peak at longer wavelengths (redder)
Hotter objects peak at shorter wavelengths (bluer)
The higher the temperature, the shorter
the peak wavelength
Very cool objects peat at radio wavelengths and very hot
objects peak at ultraviolet, x-ray, or gamma-ray wavelengths
Temperature Matters!
• The energy emitted directly proportional to
Temperature
4
• As stars get hotter, their energy output
increases quickly!
• A star 10 times hotter than Sun has
10x10x10x10 =10,000 times more energy
output
We need to define TEMPERATURE!
• Fahrenheit (used in the U.S.)
• Centigrade/Celsius – based on the freezing
and boiling points of water
– water freezes at 0 C
– water boils at 100 C
• Kelvin temperature scale
– 0 K (-273 centigrade) is the coldest temperature
possible (absolute zero)
– water freezes at +273 K
– water boils at +373 K
– room temperature 300 K
Hotter thermal radiators are brighter
at all wavelengths of light
8000 K
Thermal Radiation
Curves
Brightness
is
7000 K
3000 K
6000 K
5000 K
4000 K
300
400
500
600
700
Wavelength (nm)
800
900
1000
How do
light and
matter
interact?
Emission
Absorption
Transmission
Transparent objects transmit light
Opaque objects block (absorb) light
Reflection or Scattering
Reflection and
Scattering in
Nature
The moon
reflects sunlight.
Moonlight
scatters off the
fog and reflects
off the lake
Dust near the stars of the
Pleiades scatters starlight
Checking Understanding….
Why is a rose red?
a) The rose
b) The rose
light.
c) The rose
d) The rose
absorbs red light.
transmits red
emits red light.
reflects red light.
For Wednesday
Chapter 6 – Telescopes
Visit Kirkwood Obs
EP1 Finished on Wednesday