1	Astro 1050 Wed. Feb. 4, 2004 Today: A bit more on Parallax (p. 134-136) Chapter 5, section 1 Homework questions
2	Distances to Stars (Parallax from pages 134-136, goes with lab this week) Distance:
3	Parallax: Really just the small angle formula
4	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.
5	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
6	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”
7	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
8	The Electromagnetic Spectrum Radio waves Microwaves Infrared Visible Ultra-violet X-Rays Gamma rays
9	Relationship between Energy and Wavelength of Light Short wavelength Þ High energy photons Long wavelength Þ Low energy photons Intensity µ total energy (per area per second) µ (# of photons per area per second) ´ (energy per photon) Example with falling rain: Amount of rain µ (# of raindrops) ´ (volume per drop)
10	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 cause chemical changes Above threshold (short wavelength) energy photons can break apart DNA molecules Number of molecules damaged = number of photons above threshold Very unlikely two photons can hit exactly together to cause damage
11	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 joule/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?)