Class (9), 2/5/04 Class highlights

Reviewed items:

R1: A pure wave is characterized by three numbers: wave length, frequency, speed; however there is one equation among them,
            speed = (wave length) x (frequency),
therefore, there are really just two independent numbers that characterize a pure wave.
[We will discuss other kinds of waves later.]

R2:  The wave length of light can vary from zero to infinity (excluding the exact zero and the exact infinity). Light of different wave lengths has different names: visible light, x ray and gamma ray on the shorter side, and infra-red light and radio wave on the longer side.

R3: Characteristics of Planck distributions: Chau showed animation of
    * Stefan-Boltzman law
    * Wien's law
As always, it is important to understand what these laws imply.


R4: the concept of "proportional" to (in the short notation of "~") is useful and important, e.g.,
    * power per area (due to light emission by a black body) ~ T^4, the Stefan-Boltzman law. 
    * Lamda-max ~  1/T, in Wien's law. The proportional constant is 2.9 x 10^-3.
    *  Force ~ acceleration in Newton's second law. The proportional constant is the rest mass, M.
    * Gravitational force ~ the product of two masses and the inverse square of the distance between them.              The proportional constant is the Newton's gravitational constant, G.
    * Energy of a photon ~ its frequency. The proportional constant is the Planck constant.
These proportional constants are important fundamental constants, which are given by experiment (or dictated by nature) which usually can not be explained by theory.

R5: Doppler effects: Chau showed animation.

    Definition of Z:
        Z = % change in wavelength, which can also be expressed in terms of frequency change.

R6: Chau also showed the animation of emitting or absorbing photons with wave lengths characteristic to an atom, due to the characteristic discrete energy levels that are allowed of an electron in an atom.


New items:

 N1: Gauge Principle and standard model (electroweak plus strong; but not unified.)

 N2: The speed of light, c, is the same for all observers moving with constant velocity with each other.

 N3: In special relativity, the factor,  square-root of [1- (v-squared/c-squared)], comes up a lot. Therefore, we would want to be able to take square root in the back-of-the-envelope style. Indeed we can.

 N4. Chau demonstrated  the steps by taking square-root of 3.