Quantum Theory Wave Particle Duality EQ How did

Quantum Theory Wave Particle Duality • EQ: How did scientists know that light could act as a particle and as a wave? Why is it important to know? Definitely

The Particle Theory of Light For years it was accepted that light travels as particles (though with little direct evidence). Largely based on my corpuscular theory of light Isaac Newton 1642 -1727

Particle • A tiny piece of anything – Atom or nucleus – Quark or gluon

The Wave theory of radiation However, this idea was overthrown after Young demonstrated beyond doubt the wave nature of light in his double slit experiment (1801).

Waves • Something that moves up and down or back and forth, an oscillatory motion – Stadium wave – Water wave – Sound wave – Pendulum n ) o i n t o ti r mo o m fo r o f ired d re equ i u eq um r r : dium edi s e av (me no m w f al tic ( o c s i pe chan agne y T Me ctrom • Ele •

The photoelectric effect However, around the turn of the 20 th century, an effect that became known as the photoelectric effect defied explanation using the wave model. Oh no!

Wave – Particle Duality OR https: //www. yo utube. com/watc h? v=Df. Pepr. Q 7 o Gc

The Wave theory of radiation • Double Slit Experiment – Historically, light has sometimes been viewed as a particle rather than a wave; Newton, for example, thought of light this way. The particle view was pretty much discredited with Young's double slit experiment

The Wave theory of radiation • Double Slit Experiment Thomas Young proved that light has wave properties when he produced an interference pattern by shining light from a single coherent source through two slits.

• Double Slit Experiment Waves Particles

Heisenberg’s Uncertainty Principle • To know the velocity and position of a quark (a part of an atom) we must measure it, and to measure it, we are forced to affect it. Uncertainty about an object's position and velocity makes it difficult for a physicist to determine much about the object. The slightest interference can cause the incredibly small particles to behave differently.

The photo-electric effect. BUT…it’s a particle (Einstein) It's been determined experimentally that when light shines on a metal surface, the surface emits electrons. Why do you think this happens?

The photo-electric effect. • Light is made up of electromagnetic waves, and the waves carry energy. So if a wave of light hit an electron in one of the atoms in the metal, it might transfer enough energy to knock the electron out of its atom.

Photoelectric Effect Metal Foil

Photoelectric Effect Metal Foil

Photoelectric Effect • As blue light strikes the metal foil, the foil emits electrons.

Photoelectric Effect

Photoelectric Effect • When red light hits the metal foil, the foil does not emit electrons. • Blue light has more energy than red light. • How could we get more energy into the red light? • Try increasing the brightness.

Photoelectric Effect

Photoelectric Effect • Well, that didn’t work! • Maybe its still not bright enough.

Photoelectric Effect

Photoelectric Effect • Still not working. • What happens with brighter blue light?

Photoelectric Effect

Photoelectric Effect • More blue light means more electrons emitted, but that doesn’t work with red.

Photoelectric Effect

Photoelectric Effect • Wave theory cannot explain these phenomena, as the energy depends on the intensity (brightness) • According to wave theory bright red light should work! ►BUT IT DOESN’T!

Photoelectric Effect • Einstein said that light travels in tiny packets called quanta. • The energy of each quanta is given by its frequency Quanta E=hf Energy Planck’s constant frequency

Photon energy The photon energy is given by the following formula (which was used by Planck): E = hf where E is the energy contained in the photon (in Joules), h is Planck’s constant (6. 63 x 10 -34 Js) and f is the frequency in hertz.

Photoelectric Effect • Each metal has a minimum energy needed for an electron to be emitted. • This is known as the work function, W. • So, for an electron to be emitted, the energy of the photon, hf, must be greater than the work function, W. • The excess energy is the kinetic energy, E of the emitted electron.

Photoelectric Effect EINSTEIN’S PHOTOELECTRIC EQUATION: - W f h = E

The photo-electric effect. Wave Particle Duality Then light is made of particles! But wait. . . what about the two-slit experiment? How can light make an interference pattern like that, unless it was made of waves. We can only conclude that light is somehow both a wave and a particle--or that it's something else we can't quite visualize, which appears to us as one or the other depending on how we look at it.

Quantum Physics in Your Life • What does quantum Physics have to do with you? • Your smart phone. Digital cameras. Definitely the little Flash cards used to store all your camera's photos and your phone's apps. But seriously, every transistor is a quantum mechanical device. As is any device, your eyeballs included, that can convert a stream of photons into an electrical signal. Modern technology, and plenty of biology, swims in a veritable sea of quantum mechanics.

EXIT TICKET • Why did increasing the brightness of the red light not succeed in emitting an electron, but why did increasing blue light increase the amount of electrons being emitted? (HINT: does increasing brightness affect amplitude, frequency, or wavelength? What does energy depend on in Planck’s equation? )