What is the nature of Part II

Last week we surmised that photons must carry momentum in spite of the fact that they are massless. Last time we learned that Hertz showed that light is an electromagnetic phenomenon, and that electromagnetic waves behave much like any other wave-- they can be reflected, refracted, diffracted and polarized. However, we also learned last time we learned that, unless energy is quantized, the radiation of a blackbody will continue to increase with frequency—a delimma dubbed the ultraviolet catastophe—forcing Planck to theorize that the “oscillators” atomic walls must have quantized energy. You also had some great questions about the continuous blackbody spectrum emitted by heated solids. We’ll (briefly) revisit that in the context of what we learn today.

The observation that electromagnetic waves could eject electrons from the surface of a metal was first made by Hertz. A simple experiment can be designed to measure the energy and intensity of the electrons ejected. Light shines on a metal plate emitting electrons The voltage on a battery can be gradually turned up until the electric field just stop the electrons from reaching the collector plate, thereby giving a measure of the kinetic energy.

The energy in the light wave is spread out uniformly and continuously over the wavefront. The maximum kinetic energy of an ejected electron is therefore timelight intensity cross sectional area of atom work function absorption coefficient which depends on the light intensity and the time over which it is exposed. The intensity of a light wave is proportional to the square of the amplitude of the electric field. The energy in the light wave is spread out uniformly and continuously over the wavefront. …and therefore does not depend on frequency.

The number of photoelectrons ejected depended on the intensity (as expected) but their maximum kinetic energy did not! The maximum kinetic energy depended only on the frequency, the slope of the linear relationship between the energy and the frequency gives “Planck’s constant”, h. The electrons were ejected immediately after the light started shining—the electron instantaneously absorbed enough energy to escape-provided there was enough energy to overcome the binding energy or “work function”. Even a high intensity source of low frequency light cannot liberate electrons.

We have to change our way of thinking about this picture: Instead of continuous waves we have to think of the energy as being localized in quanta. In the photoelectric effect, these discrete localized quanta of energy, hv, are transferred entirely to the electron

Part 2: Compton scattering: when you have a higher energy photon Photoelectric effect- all of the incident photon’s Energy is transferred to an electron, ejecting it. Compton scattering- electron is ejected, but photon retains some energy. Pair-production-the photon’s energy is consumed to produce an electron and a positron.

The unshifted peak comes from tightly bound electrons Contrast, classical scattering: Electrons would shake with the frequency of the incident wave The incident and scattered wavelengths would be the same

Bragg spectroscopy (a)Constructive interference occurs when: (b)At other angles the waves do not interfere constructively This is an important tool in crystallography as it is a sensitive measure of the spacing of the crystalline planes.

…and the answer is…drumroll please… If light, which was previously thought of as a wave, has characteristics of particles, could it be true that particles must also be thought of as waves in some contexts in order to fully describe their behavior? Light: Interferes like a wave Diffracts like a waves Can be polarized like a wave Scatters like a particle