Astronomy 1 - Lecture Notes Monday - October 5, 1998

Announcements Astronomical  :Full Moon 20 hr UT = 17 hr ADT = 5 pm ADT
		('Harvest Moon Exercise') - Exercise 5 Start now - Due Oct 14
		Tuesday Moon at Perigee (357,633 km)
			compare 384,000 km semi-major axis
			Large Tides next few days!!!
Homework  - Read All of Chapter 3 on Electromagnetic Radiation
		Exercise 4 due on Wednesday this week
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OTHER TYPES OF ORBITS
	Escape Velocity and Orbital Energy (See Sept 30 Lecture)
	Parabolic
	Hyperbolic
ELECTROMAGNETIC RADIATION
Electric Fields
	
Magnetic Fields
	
Electromagnetic Wave from an Antenna
	
	- Propagation of Electromagnetic Wave - Velocity of Light
			c = 300,000 km/s = 3x108 m/s
	- Wavelength, Period (frequency), Speed
			(See figure 3.8)
		Units of frequency = Hz = 1 cycle/second
		Period of Oscillation = 1/frequency 
		velocity of the wave = wavelength x frequency
		For Light    
		f = frequency,  lambda = wavelength 
SPECTRUM OF ELECTROMAGNETIC RADIATION
	(See figure 3.11)
	-Radio			AM 1MHz : l= 3x108/1x106 = 300 m	
				FM  100 MHz: l = 3x108/1x108 = 3 m
	-Microwaves	l = 3 cm      f = 100*100MHz = 10 GHz
		MHz = 106 Hz		GHz = 109 Hz
	-Infrared	l = 10 mm
		nanometre = 10-9 m	
	-Visible Light	l = 400 - 700 nm
		violet, blue,green, yellow, orange, red
	-Ultraviolet	l = 100 nm
	-X-ray		l = 1 nm	(E = hf/e = 50,000 eV = 50 KeV)
		Electron Volt = eV = 1.6x10-19 Joules
		(Energy of 1 electron accelerated through one Volt potential)
		Kev = 1000 eV 		MeV = 106 eV
	- Gamma rays	 l = 0.01 nm		E = 5 MeV

Intensity of Light = Power/area
	Power = Energy/time 	[power] = J/s = Watt, W
	Intensity propotional Amplitude squared 
		(E = amplitude of electric field)
	Light Intensity ~ E2
	Corpuscular Theory of Light --> Photons
		Energy of a photon = hf
		h = Planck's constant = 6.63x10-34 J.s
		Energy = hc/l 
-->	Energy (eV) = 1240/l(nm)

		Visible light = 500 nm (green)
		photon energy = 1240/500nm = 2.48 eV

Inverse square law of intensity for isotropic light
	
	Light spreads out over a sphere of area 4(pi)d2
-->	I ~ 1/d2
	
EXAMPLE: Sunlight and Star Light
	- Solar Constant = intensity of sunlight at the Earth
		= 1370 Watts/m2	
	What is the Power Output of the Sun?
		R = distance = 150,000,000 km = 15x1011 m
		Area of Sphere at Earth's Orbit = 4(pi)R2
			= 2.8x1023 m2
		Total Power across sphere = (1.37x103)(2.8x1023 m2)
			= 3.9x1026 Watts
			= Power output of the Sun
	What would be the intensity of the Starlight from a Sun-like star at a
	distance of 5 light-years? 1 light-year = 9.46x1015 m
		Intensity = 4x1026W/(4 x 3.141)(5 x 9.46x1015 m)2		
			= 1.8 x10-8 W/m2
	How much does the Eye see?
		Pupil area (7 mm diameter) = 4 x 3.141 x (3.5x10-3)2		
				= 1.54x10-4  m2
		Power seen from star = 1.54x10-4  m2 x 1.8 x10-8 W/m2
			= 2.8x10-12 W
		How many photon/second (wavelength = 500 nm) E = 4x10-19 J

		number/second = (2.8x10-12 J/s)/(4x10-19 J)
			= 7x106 per second

Thermal Radiation 
	- Temperature Scale = Absolute (Kelvin)
			T(K) + 273 = T(C)
	- Black Body Radiation Distribution (Spectrum)
-->		Total Intensity = I (W/m2)= s e T4
			e = emissivity
			s = Stephan Constant = 5.67x10-8 W/m2/K4
	Example: Human Body at 310 K (area  ~ 2 m2)
		Power = 5.67x10-8(300)4 2 m2
			= 500 x 2 = 1000 Watts
		emissivity ~ 0.2  ---> 200 Watts

	Wein Displacement Law - Wavelength of maximum intense thermal radiation
-->		lmax = 2900 mm/T(K)
		l = wavelength
	Human Body 300 K   lmax = 10 mm (Infrared, IR)
	Light Bulb 2000 K	lmax = 1.45 mm = 1450 nm (near IR)
	Solar Surface 5800 K	lmax = 0.50 mm = 500 nm (visible light)
	Very Hot Star 60,000 K     lmax = 50 nm (ultraviolet)

Doppler Effect
	Motion of Source, with speed 
	viewer<) ----------------- O-->v
					red shift, dl/l ~ v/c
			dl = lapparent - lsource
	viewer<) -------------- <--O
			v		blue shift, dl/l ~ - v/c
v = relative velocity (either the source or the observer may be moving)
Example:
	small effect Earth orbital speed = 30 km/s
	v/c = 30/300,000 = 10-4
	lsource  = 500 nm
	dl = 500 nm x 10-4 = 0.005 nm

The eye and Light
	Spectral Response - 
		Color Vision (cones)   550 nm peak
			Most in fovea - central vision of eye
			less sensitive but more resolution
		Black and White Vision (rods)  500 nm peak
			periperal vision - 
			more sensitive but less resolution
	Dark Adaptation ~ 1/2 hour
		iris adjust and rhodopsin restoration
		red light effects dark adaptation less - red filtered flashlights
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