Astro 1050 Wed. Feb. 4, 2004

Today: A bit more on Parallax (p. 134-136)

Chapter 5, section 1

Homework questions

Distances to Stars
(Parallax from pages 134-136, goes with lab this week)

Distance:

Parallax: Really just the small angle formula

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 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)

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

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?)


	Today: A bit more on Parallax (p. 134-136)
	Chapter 5, section 1
	Homework questions


	Properties of light are fundamental

	Almost everything we know about the universe outside our solar system comes from interpreting the light from distant objects.


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


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”


	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


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


	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)


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


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?)