Electromagnetic Spectrum
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Visible light and color
Energy, frequency, wavelength
A bit on telescopes
Uses
Why do we care about light?
• Everything we know in astronomy
comes from light
• We only have physical samples from
the Moon, Mars, and a couple comets
– Even these we study using light
Electromagnetic Radiation
• Radiation: when things are sent out
into empty space
• EM radiation: light
• Nuclear radiation: result of nuclear
reactions, includes
– Alpha particles (Helium nucleii)
– Beta particles (positrons = anti-matter
electrons)
– Gamma “particles” (gamma EM radiation)
What is Light?
• Electric and magnetic fields waving
• Packets of energy called photons
• Energy and wavelength determine what
the light can interact with
http://www.yorku.ca/eye/spectrum.gif
False Color Images
• Pictures attempt to “translate” invisible
light to what we can see.
• Representative color – colors have
meaning, such as specific non-visible
colors or chemicals
• “Touch the Universe” by Noreen Grice
High Energy
High Frequency
Short Wavelength
High Energy
High Frequency
Short Wavelength
• Interacts with small things (cells,
molecules, DNA)
• Created by high energy / hot processes
http://upload.wikimedia.org/wikipedia/commons/9/99/EM_Spectrum3-new.jpg
ϒ-ray (gamma) and X-ray
• High energy, short wavelengths
• On Earth: radioactivity, some cancer treatment,
broken bones, cavities
• Astronomy: high energy processes, hot environments,
colliding stars, around black holes, gas around
galaxies, the Sun
http://today.slac.stanford.edu/images/2007/swiftgamma-ray-lg.jpg
http://www.atnf.csiro.au/people/mdahlem/img/xmm
tel.gif
Ultraviolet (UV)
• Slightly higher frequencies
and shorter wavelengths
than humans can see
• Some birds and insects can
see in the UV
• On Earth: suntans, sunburn,
cancer, can be used to
sterilize equipment
• Astronomy: Used to study
the Sun and other hot
objects
http://sohowww.nascom.nasa.gov/data/realtime/eit
_195/1024/latest.html
– Hubble is mostly visible, but
does have one UV instrument
Visible light and color
• A small part of the total spectrum
• Approximately 400-700nm, 40007000Å, violet to red
• Each color the eye can see has a
different wavelength.
http://solarsystem.nasa.gov/deepimpact/images/Spect-Prism-sm.jpg
Why do our eyes see with
Visible light?
Atmospheric Windows
Wavelength,
Energy
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
Why do our eyes see with
Visible light?
• Atmospheric windows
– Visible and Radio get through an Earth-like
atmosphere
• Wavelength determines size of things it
can interact with
– Due to evolution, sizes of our eyes, sizes
of molecules in our eyes, we see visible
light
Low Energy
Low Frequency
Long Wavelength
Low Energy
Low Frequency
Long Wavelength
• Interact with larger things
• Lower energy processes
http://upload.wikimedia.org/wikipedia/commons/9/99/EM_Spectrum3-new.jpg
Infrared (IR)
• Slightly longer wavelength and lower frequency than the human
eye can see.
• Some animals including some snakes, some bats, and some
insects can see IR.
• On Earth: IR goggles help firefighters, soldiers, and home
insulation experts; TV remote controls, Kinect
• Astronomy: IR travels through dust so is good to look at young
forming stars. (e.g., James Webb Space Telescope, HST’s
successor)
http://upload.wikimedia.org/wikipedia/en/c/cf/Ir_gir
l.png
Microwave/Sub-mm/Radio
• Low energy, long wavelength
• On Earth: microwave ovens, cellphone signals, radio
for stereos, (formerly) TV broadcasts, GPS, walkytalkies, satellite communications, GPS, weather radar,
radar guns
• Astronomy: cool objects such as clouds of dust, gas
clouds, entire galaxies, SETI, Cosmic Microwave
Background (light from the Big Bang)
http://apod.nasa.gov/apod/image/0302/sky_wmap_big.j
pg
http://cache.eb.com/eb/image?id=96170&rendTypeId=4
Bad Science: “Listening” to Radio
Like all types of light, radio light is studied
using either images or spectra (graphs of
the “colors” – brightness at each
Nancay Radio Telescope
http://upload.wikimedia.org/wikipedia/commons/9/99/EM_Spectrum3-new.jpg
Each part of the
wavelength tells us
part of the story.
Must put together all
wavelengths to get the
full story.
http://www.thunderbolts.info/tpod/2006/image06/060123
Example: The Sun
• http://sohowww.nascom.nasa.gov/data/realti
me/realtime-update.html
• Putting these together helps us understand
the layers of our dynamic Sun.