Astro 1050     Mon. Feb. 7, 2005
   Today:  Review exam 1
Chapter 5

Chapter 5: Astronomical Tools
Properties of light are fundamental
Almost everything we know about the universe outside our solar system comes from interpreting the light from distant objects.

Radiation: Two different kinds
Something that “radiates”, or spreads out in “rays”
High speed particles (eg. high speed neutrons ejected from a disintegrating atomic nucleus)
Electromagnetic radiation:
Towards shorter “wavelength” and higher energy:
Visible light, Ultraviolet light, X-Rays, Gamma-Rays
Towards longer “wavelength” and lower energy:
Visible light, Infrared radiation, microwaves, radio waves

Properties of Light
Light has both wave and particle properties
Travels like a wave
Interacts with matter like a particle:  “photon”
Full explanation involves quantum mechanics
For most cases we can just choose the right “model” from the above two choices
Photons, unlike particles in other kinds of “radiation,” are particles of “pure energy”

Light is an electromagnetic wave
Changing electric fields generates magnetic fields
Changing magnetic fields generates electric fields
Can set up a cycle where one field causes the other:
The E and B fields oscillate in strength, and the disturbance moves forward.
To describe the wave you need to specify
Direction it is moving
Strength of the fields (its intensity)
Frequency or Wavelength of the oscillation (u and l are inversely related)
Orientation of the electric E field:  up or sideways (polarization)
You do not need to specify its speed
In a vacuum all lightwaves move at the same speed c = 3´108 m/s

The Electromagnetic Spectrum
Radio waves
Microwaves
Infrared
Visible
Ultra-violet
X-Rays
Gamma rays

Relationship between Energy and Wavelength of Light
Short wavelength Ţ  High energy photons
Long  wavelength Ţ Low energy  photons
Intensity µ total energy (per area per second)

              
µ     (# of photons per area per second)
                      
´  (energy per photon)

Why is energy per photon so important?
Modified example:  Hailstorm (with your car outside in it)
Threshold for damage to car set by size of individual hailstones
Below threshold hailstones cause no dents
Above threshold they cause bigger dents
Number of dents = number of hailstones bigger than threshold
Very unlikely two small hailstones can hit exactly together to cause dent
Real life example:  Ultra-Violet light hitting your skin
Threshold for chemical damage set by energy (wavelength) of photons
Below threshold (long wavelengths) energy too weak to break bonds
Above threshold (short wavelength) energy  photons break apart DNA
Number of molecules damaged = number of photons above threshold
Very unlikely two photons can hit exactly together to cause damage

Numerical Relationship between
 wavelength and photon energy
Inverse relationship:  Smaller l means more energetic
c = speed of light = 3.00 ´ 108 m/s
h = Planck’s constant = 6.63 ´ 10-34 joules x s
Note:  Joule is a unit of energy        1 Joule/second = 1 Watt
Energy of a single photon of 0.5 mm visible light?
Seems very small, but this is roughly the energy it takes to chemically modify a single molecule.
Photons from a 100 W lightbulb  (assuming all 100W goes into light?)

Optical Telescopes and Cameras
Properly curved lenses and mirrors can form “Images”
All the light leaving one point on object gets “reassembled” at one point on the image.

Refracting vs. Reflecting Telescopes

Why do astronomers need large telescopes?
Large telescopes can collect more light
Can detect fainter objects
Have more light for specialized analysis.
Large telescopes can form more detailed images
Waves spread out as they go through an opening.
The larger the opening, the less they spreads out.
The longer the wavelength the more they spread out
Angle of spread q µ l/D  where D is Diameter of telescope
Radio telescopes have to be much bigger than visible ones

Kinds of measurements made with telescopes
Measure brightness of objects (photometry)
Record images using electronic “CCD” detectors
Split it into different wavelengths with “spectrometers”

Dark Side of the Moon
“There is no dark side really.  It’s all dark.” -- Pink Floyd

Dark Side of the Moon
What is wrong with this picture?
Front: Not all primary colors (eg, pink, magenta), also refraction angles inconsistent
Back: Spectrum is Convergent – I think done for art’s sake

Observing over the entire electromagnetic spectrum
Different phenomena produce different wavelength waves
Ordinary stars:  Visible light
Cool planets or dust clouds:  Infrared light
Moving charged particles, cool molecules:  Radio waves
Very hot objects:  X-Rays and Gamma Rays
Quasars: ALL wavelengths
Only visible, some IR, and radio make it through atmosphere
Need to observe from space for other wavelengths
Going into space also lets you obtain more detailed images
On Earth telescope size isn’t only limit on image resolution
Temperature fluctuations in atmosphere cause “seeing” (blurring)

Bad “seeing”/Good “seeing”

Active/Adaptive Optics

Hubble Space Telescope (HST)

Chandra X-ray Observatory

Radio Telescopes

Infrared Telescopes

Infrared Telescopes

The Electromagnetic Spectrum
Radio waves
Microwaves
Infrared
Visible
Ultra-violet
X-Rays
Gamma rays