A105
APOD
 Homework 3 due Sept. 14
 EM Radiation
 Read units 21, 22, 23, 24, 25
 News Quiz TODAY
 Observing the Full Moon

Last
Thursday
Orbits between the Sun and Earth, about a million miles away – a constant view of the Sun
Sunday
Today

What does “electromagnetic” mean?

Wavelength and Frequency

 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
W

340 ( m / sec)
262 (/ sec)

1 .
2 ( meters )

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 10 8 meters per second)
 frequency – say, one billion cycles per second
(10 9 cycles per second)
 What is the wavelength?
 What kind of light is this?
3 x 10 8 m/sec = W x 10 9 /sec
W ( meters )

3 x 10
8
( m / sec)
10
9
(/ sec)

0 .
3 ( meters )

 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

 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

• 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

 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 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

X-Ray
Multiwavelength
Orion
Optical
UV
Radio
Infrared

X-Ray
UV
Visible
The Multiwavelength Sun
Infrared
Composite
Radio

Venus Multiwavelength
UV
Visible
Infrared
Radio

A Mnemonic for the
Electromagnetic Spectrum…
– Gamma Rays
– X-rays
– Ultraviolet
– Visible
– Infrared
– Microwave
– Radio
 Really Messy Insane Valentine
Unlike (ex) Girlfriend
 Reindeer Make Interesting
Visitors Uttering X-mas
Greetings
 Great X-rated (Unacceptable)
Videos In My Room
 Roy Marches In Very Unique Xray Glasses
 Randy Makes Interesting Voices
Under Xtreme Gasses

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
Warmer matter glows with higher energy light with light
Even hotter matter glows blue hot
Matter at about
10,000 degrees centigrade 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

• 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

is
Hotter thermal radiators are brighter at all wavelengths of light
8000 K
Thermal Radiation
Curves
7000 K
6000 K
3000 K
300
5000 K
4000 K
400 500 600 700
Wavelength (nm)
800 900 1000

Why don’t we glow in the dark?
a) People only reflect light.
b) People emit infrared light that is invisible to our eyes.
c) People are too small to emit enough light for us to see. d) People do not contain enough radioactive material.

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