Mr. Thompkins in Wonderland Read here

Quantum nature of light (photons) When you turn the lights down, the most sensitive detectors see this:

Photoelectric effect in atoms, metal First experiment of how light interact with electrons. We get a range of electron speeds, but we’ll focus on the fastest that come out.

Photoelectric effect in atoms, metal What happens to electron speed when we turn up the light intensity (greater E/M wave amplitude)? Classical expectation: electrons will go faster

Photoelectric effect in atoms, metal What happens to electron speed when we turn up the light intensity? Experiment: no faster, just more of them

Photoelectric effect in atoms, metal What happens to electron speed when we change color of light and keep intensity the same? Classical expectation: no change in speeds

Photoelectric effect in atoms, metal What happens to electron speed when we change color (frequency) of light ? Experiment: frequency is the only thing that the fastest electrons’ speeds. Below a minimum frequency, no electrons come out

Photoelectric effect in atoms, metal http://phet.colorado.edu/en/simulation/photoelectric Experiment: frequency is the only thing that affects electron speeds.

Experiment explained by photons Light only comes in chunks of energy called photons Why is there a minimum frequency of light to eject electrons? The metal has minimum “binding energy” with which it hold the electrons in.

Suppose we have two laser beams emitting the same power of light (energy/time) hitting a screen. P1. Which beam has the most photons hitting the screen per time? A. infrared (hf = 2.0 eV) B. blue (hf = 3.0 eV) C. neither…same P2. Which beam has the most force (momentum per time) on the screen, assuming all the photons are absorbed?

In the photoelectric effect with a metal with binding energy 0 In the photoelectric effect with a metal with binding energy 0.8 eV, you use three light beams: infrared (0.5 eV), red (hf = 2.0 eV) blue (hf = 3.0 eV) P3. Which beam creates the most emitted electrons? A. very intense IR B. intense red C. weak blue P4. Which beam needs the highest stopping voltage? P5. What is the stopping voltage for electrons ejected by red light? ___ V A. 0.5 B. 1.2 C. 2.0 D. 2.2 E 3.0 What is the longest l that will eject electrons?

We can’t know where each photon will go, but each photon lands in a place with probability proportional to the classical intensity expected or the square of the classical electric field of light.

Wave nature of light Two-slit experiment with intense light. What happens with very low intensities (photons)?

Wave nature of light particles Two-slit experiment. Turn light down so only one photon can be at the slits at a time. Do we see diffraction? Yes!...statistically. Does a single photon “go through both slits ” or just one?

Wave nature of light particles These results challenge the common statement: “Wave and particle aspects of light are never manifest at the same time”. They certainly can be seen in the same experiment! Many photons together make up a classical light wave.

X-ray production Opposite of photoelectric effect: high energy electrons create photons Use thousands of volts to accelerate electrons

X-ray production We see a range (spectrum) of energies of x-ray photons produced, up to the energy (qV) of the electrons that created them (metal binding energy can be neglected as small compared to qV).

X-ray production Smooth part of spectrum: Bremsstrahlung (“braking” radiation of x-rays)

X-ray sharp lines Sharp peaks (used most in x-ray machines) “core electrons” knocked out by incoming electrons, then others jump into “hole”, giving off x-ray

X-ray sharp lines The lines’ energies depend on the metal used.

P7. If I increase the voltage on an x-ray machine the sharp peaks ____ P7. If I increase the voltage on an x-ray machine I will be able to create x- rays with ____ A. longer l B. shorter l C. neither: l doesn’t change   P7. If I increase the voltage on an x-ray machine the sharp peaks ____ A. move to longer l B. move to shorter l C. don’t move P8. If an electron and antielectron (positron) annihilate, the photons produced will be _________________ compared to a proton and antiproton annihilating.