Friday January 22nd 1999
Announcements:

Lecture notes:
Class 6.

Today we will answer several important questions

• How is light made?  How fast is light?
• How can light have different colors?
• How do we perceive colors and what do they mean in terms of properties of light?
• Is there "light" beyond the end of the rainbow that we can't see?
• How to we detect radiation that is invisible to our eyes?

How is light made?
made when particles with an electric charge move around (elections)
light from galaxies is made when charged particles in these galaxies move around.

In a rough sense you can see objects with higher temperatures will emit more energy.  Charged particles in a system with high T will move more and thus make more light

Also of course larger things can emit more light than smaller things.  More charged particles to move around.
 

How fast is light?
light in a vacuum always travels at the same speed no matter what color it is
speed of light is 30 billion cm/sec = 3x10¹⁰ cm/s
7.5 times around the Earth in one second
500 000 times faster than a fighter jet at Mach 2
called c a fundamental constant of the universe the ultimate speed limit

Strictly speaking light is not a wave and not a particle.  Light is its own unique phenomenon and it behaves by its own rules.  However light does have aspects that are wavelike and particle
like.  In some ways talking about light as a wave or particle is very useful although if taken too literally confusion can result.

In this course we will speak in rough terms using loose wave and particle analogies.

The wave-like nature of light
Often useful to think of light as a wave.
Can think of light similar to water waves traveling away from an impact point.
Amplitude - how high peaks are compared to the valleys
Wavelength - distance between two peaks
Frequency - as a wave moves by this is the number of peaks that pass a given point in 1 second.

Now hot things have lots of energy in them and charged particles move fast thus high frequency and short wavelength.
So can use light to learn temperature.  Short wavelength means hot objects

The particle nature of light
photon = particle of light
can count photons
high energy light acts more like particles like x-rays and gamma rays

How do we see light?
makes particles in our eyes move
light from distant galaxies can travel billions of years and they still jiggle particles in our eyeballs

Colors of light - the spectrum
blue light = high frequency short wavelength
red light = low frequency long wavelength

wavelength of visible light is 4x10^-5 cm(blue)  to 7x10^-5 cm (red)

light we can see with our eyes is only a small part of the electromagnetic spectrum of light
astronomers use all these types of light to learn about the universe
We have different machines to sense these types of light
Dr. Brandt is an X-ray astronomer
 

• IR. radiation = slightly longer wavelength than red seen in embers after a camp fire.  They cool off but still emit heat in the form of IR.
• UV radiation = slightly shorter wavelength than blue causes sunburn some is absorbed by ozone layer
• microwaves = microwave ovens
• radio waves = FM 2.8-3.4 meters.  AM 200-600 meters X and gamma rays - dentist penetration seen from black holes