Quantum Theory Chapter 27 According to Hertz Maxwells theory showed that all of optics was explained by electromagnetic theory, but…….

Two small problems remained 1. Wave theory could not describe the spectrum of light emitted from a hot body, such as steel in a photo. 2. Ultraviolet light discharged electrically-charged metal plates.(this was called the photo electric effect) Solving these two little problems required the development of quantum theory.

) These so called “hot” bodies produced such thermal energy(from vibrating particles) that they glowed.(radiate) The hotter they become the more energy they put off. And the brighter the light becomes. The frequency at which the maximum amount of light is emitted is directly proportional to the temperature in Kelvins. As temp increases so does power. The amount of power emitted each second is proportional to the absolute temp raised to the 4th power.

Look as the spectrum on page 556 of your text. Ex. Our sun is a yellow star is has less power and lower temp than a white star. This is true even though our sun radiates each square meter of Earth with 1000J per second. Look as the spectrum on page 556 of your text. Max Planck assumed the energy of all vibrating atoms gave off specific frequencies and introduced this revolutionary equation in 1900. E = nhf f= frequency of vibration h= Plancks constant (7X10 -34 J/Hz) n= must be an integer

The energy of an incandescent body is quantified, Max Planck Planck also stated ”All atoms do not radiate electromagnetic waves when they are vibrating, instead they only emitted when their vibration energy underwent a change” He also put forth that the changes only occurred in integer valued "quanta” ie. The change in whole values of hf=E This was the first time the scientific community realized that the physics of Newton was not valid on all conditions.

Photoelectric effect By definition: The emission of electrons when electromagnetic radiation falls upon a material. (more on page 557) Regardless of how bright or dim the light is, to allow any emission of electrons the energy (light) on a metal must exceed the threshold frequency. Fo. This threshold frequency varies with each metal.

Recall that electromagnetic radiation theory states, the more intense the radiation, regardless of the frequency, the stronger the electric and the resulting magnetic field. Thus for low intensity radiation the electrons would need to absorb radiation for a long period of time to reach the threshold frequency and be ejected. The photon theory on the other hand explains the photoelectric effect by proposing that light and all other forms of radiation consist of discrete bundles of energy (Quanta) and the energy of each photon depends on the frequency of the light.

In comes Einstein In 1905 Einstein stated, “the light and other forms of radiation must come in discrete bundles of energy, later named photons. The energy of each photon would depend on the frequency of the light. He explaind the existance of the threshhold Planck spoke of and stated that the excess energy not used to release electrons was shown in the formula KE= hf-hfo yes, kinetic energy

How can Einstein theory be tested How can Einstein theory be tested? Using a cathode tube like the on pictured on pg 558. The max KE of the electrons at the cathode is equal to at the anode KE = -q Vo Vo is the magnitude of the stopping potential in J/C and q is the charge of an electron in C. Since a Joule is so large for atomic systems we use electron volts. 1eV =1.6x10-19J

A graph of the kinetic energies of electrons ejected from a metal verses the frequencies of the incident photons is always a straight line. If you put max KE on the y-axis and Frequency on the x-axis of a graph the slope of the line yields Planck constant (h) The graph differs for each metal but always produces the same results. The threshold frequency needed to release the most weakly bound electrons is known as the work function. W= hfo

The Compton Effect Even though a photon has no mass it has kinetic energy? ρ = hf/c =h/λ why? c is the velocity of light and v= λf Yes, this is showing that you can indeed have momentum without mass, neat huh?

The energy of a photon is given by E= hc/ λ If a wave length increases the photon loses both energy and momentum. This means that a tiny shift in wave length in scattered photons can cause a loss of momentum/energy. This energy is conserved by the electrons in the metal in equal amounts to what is lost by the photons. Thus, Compton prove that even though Photons have no mass they do obey the conservation of energy theorem.

Assign page 570 reviewing concepts 1-10 all, Problems 1-13 same page.

A particle that behaves like a wave? Victor de Broglie in 1923 showed that electromagnetic waves had particle like properties. Since ρ = mv = h/λ λ= h/ρ= h/mv Remember KE= qV= 1/2mv2

Wrap up This little “non-mass” particle exhibits diffraction and interference and it spreads out like a wave. Electromagnetic partials called photons act just like what Einstein theorized light would, both as a wave and a particle and thus exhibit duality. SORT OF LOKE HAVING YOUR CAKE AND EATING IT TOO!T However this one is special, it has no mass but exhibits both momentum and energy?

The Heisenberg Uncertainty principle The Heisenberg Uncertainty principle states that one cannot ever know exactly the location and momentum of a photon of light. It simply cannot ever be exactly measured. Why? 1. you measure where a particle of light is 2. by the time you decide its placement is, ,it has moved. COOL HUH? Assign pg 571 14-21 all