PHYS 182 Lecture Notes - Lecture 7: Radio Wave, Black Body, Atomic Orbital
PHYS182: Our Evolving Universe
2017-09-26 LEC 7:
Midterm: responsible for what is covered in class + the readings
à Don’t worry about Chapter 3
Light and other Radiation
• This is how we explore the universe; we probe the universe with light
• Discuss the instruments we use; i.e. telescopes
• What have we learned about the universe with these tools?
First part of Ch.2 was motion, energy, gravity
We ended with black holes
Why you take matter like the sun, and fi you compress it enough you get a black hole, from which light
cannot escape
2.2 Light and Matter
1 – Light
• What is light? An electromagnetic wave (electric & magnetic fields)
o It is a wave in a field
• An electric field is a vector: E (x, t) (bold is a vector)
• Magnetic field is also a vector: B (x, t)
Scalar field:
Imagine you have a table in the presence of the gravitational acceleration of the Earth, and at each point
on the table, I pin down the top of the pencil, but the back end is free to rotate back and forth.
The back ends of the pencils are connected by springs.
• The pencils are connected to the table at point X
• The angle φ from the vertical
No light: E = B = 0
Light: wave-like excitation of E and B (i.e. not = 0)
Light wave
Sound wave
Fundamental
Propagates in a vacuum with no matter
Not tied to a medium
Exist in a vacuum
Effective
This is a pressure wave in a medium
Tied to a medium
Don’t exist in a vacuum
Experiment: take a chamber with a light bulb and a bell in the middle
• The light bulb is on and the bell is ringing
• Now, take out all of the air in the chamber à turn it into a vacuum
o No more air = no more sound
• As you pump out the air, the sound level will go down but the light will be unaffected
Characterizing Light
A wave is an excitation of the electric field that travels at the speed of light (c)
C = 3x105 km/s
What characterizes the wave: amplitude (brightness), wavelength (colour), frequency f = c / wavelength
Document Summary
Midterm: responsible for what is covered in class + the readings. First part of ch. 2 was motion, energy, gravity. Why you take matter like the sun, and fi you compress it enough you get a black hole, from which light cannot escape. It is a wave in a field: an electric field is a vector: e (x, t, magnetic field is also a vector: b (x, t) (bold is a vector) The back ends of the pencils are connected by springs: the pencils are connected to the table at point x, the angle from the vertical. Light: wave-like excitation of e and b (i. e. not = 0) This is a pressure wave in a medium. A wave is an excitation of the electric field that travels at the speed of light (c) What characterizes the wave: amplitude (brightness), wavelength (colour), frequency f = c / wavelength. Example: wavelength = 1 cm, gives a frequency of 3x1010 hz (1/s) = 30ghz.
