© 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 31: LIGHT QUANTA

© 2010 Pearson Education, Inc.

Conceptual Physics11th Edition

Chapter 31:

LIGHT QUANTA


This lecture will help you understand:• Birth of Quantum Theory

• Quantization and Planck’s Constant

• Photoelectric Effect

• Wave–Particle Duality

• Double-Slit Experiment

• Particles as Waves: Electron Diffraction

• Uncertainty Principle

• Complementarity


Birth of Quantum Theory• There has been a long historical debate about the

nature of light:– Some believed it to be particle-like.– Others believed it to be wavelike.

• Young’s double-slit experiment in 1801 proved that light was a wave.

• Max Planck in 1900 hypothesized that radiant energy was emitted in discrete bundles, each of which he called a quantum.


Quantization and Planck’s Constant• Quantum physics states that in the microworld of

the atom, the amount of energy in any system is quantized—not all values of energy are possible.– Example: The energy in a beam of laser light, which is a

whole-number multiple of a single lowest value of energy—one quantum

• The quanta of light, and of electromagnetic radiation in general, are the photons.

• Energy of a quanta:– E = hf where h is Planck’s constant


The Photoelectric Effect

Quantization• The idea that the natural world is granular rather

than smoothly continuous

Quantum• Any elemental particle that makes up matter or

carries energy



The photoelectric effect• A model for how matter radiates

– Hypothesized by Max Planck, a German theoretical physicist in early 1900s

– Warm bodies emit radiant energy (light) in individualized bundles (quanta).

– Energy in each quantum is proportional to the frequency of radiation.

• E ~ f, or with Planck’s constant h, E = hf


The Photoelectric Effect• Light shining on the negatively charged, photosensitive metal

surface liberates electrons.• The liberated electrons are attracted to the positive plate and

produce a measurable current. • If we instead charge this plate with just enough negative

charge that it repels electrons, the current can be stopped.• We can then calculate the energies of the ejected electrons

from the easily measured potential difference between the plates.



The photoelectric effect (continued)



The photoelectric effect (continued)



The photoelectric effect

• Einstein’s view on light– As a stream of particles, bundles of energy (photons).– Photons interact with matter one at a time.– High-energy photons dislodge electrons from certain

metals.


In the photoelectric effect, the brighter the illuminating light on a photosensitive surface, the greater the

A. velocity of ejected electrons.

B. number of ejected electrons.

C. Both A and B.

D. None of the above.

The Photoelectric EffectCHECK YOUR NEIGHBOR


In the photoelectric effect, the brighter the illuminating light on a photosensitive surface, the greater the



C. Both A and B.


The Photoelectric EffectCHECK YOUR ANSWER


In the photoelectric effect, the higher the frequency of the illuminating light on a photosensitive surface, the greater the



C. Both A and B.


The Photoelectric EffectCHECK YOUR NEIGHBOR


In the photoelectric effect, the higher the frequency of the illuminating light on a photosensitive surface, the greater the



C. Both A and B.


The Photoelectric EffectCHECK YOUR ANSWER


Wave–Particle Duality

Wave–particle duality• A photon behaves as a particle when emitted by

an atom or absorbed by photographic film or other detectors.

• But it behaves as a wave in traveling from a source to the place where it is detected.

• In this sense, light can be both a wave and a particle!


Wave–Particle DualityWave–particle duality (continued)

• This image is built up photon by photon.


Double-Slit ExperimentDouble-slit experiment

• Monochromatic light passing through two slits, a, forms an interference pattern, b, shown graphically in c.


Double-Slit Experiment• Suppose we dim our light source so that, in effect,

only one photon at a time reaches the barrier with the thin slits.

• If film behind the barrier is exposed to the light for a very short time, the film gets exposed as shown below.– Each spot represents the place where the film has been

exposed by a photon. – If the light is allowed to expose the film for a longer time,

a pattern of fringes begins to emerge


Double-Slit Experiment• If we cover one slit so that photons striking the

photographic film can pass only through a single slit, the tiny spots on the film accumulate to form a single-slit diffraction pattern.

• We find that photons hit the film at places they would not hit if both slits were open.


Double-Slit ExperimentHow do photons traveling through one slit “know” that the other slit is open and avoid certain regions, proceeding only to areas that will ultimately fill to form an interference pattern?• Each single photon has wave properties as well as particle

properties. • The photon displays different aspects at different times. • A photon behaves as a particle when it is being emitted by an atom or absorbed by photographic film or other detectors, and behaves as a wave in traveling from a source to the place where it is detected.• So the photon strikes the film as a particle but travels to its position as a wave that interferes constructively.


Particles as waves: electron diffraction• Every particle of matter is associated with a

corresponding wave. According to Louis de Broglie, a particle’s wavelength is related to its momentum.

Particles as Waves: Electron Diffraction

momentumWavelength h


When we speak of de Broglie waves, we’re speaking of the wave nature of

A. transverse waves.

B. longitudinal waves.

C. particles.

D. quantum uncertainties.

Particles as Waves: Electron DiffractionCHECK YOUR NEIGHBOR


When we speak of de Broglie waves, we’re speaking of the wave nature of

A. transverse waves.

B. longitudinal waves.

C. particles.

D. quantum uncertainties.

Particles as Waves: Electron DiffractionCHECK YOUR ANSWER



Electron diffraction• Interference patterns of beams of light

(left) and electrons (right) compared



Electron microscope uses the wave nature of electrons to create images similar to the image of the mosquito shown here.


Uncertainty Principle

Uncertainty principle• The act of observing something as tiny as an

electron probes the electron and, in so doing, produces a considerable uncertainty in either its position or its motion.



Uncertainty principle (continued)

• German physicist Werner Heisenberg called this the uncertainty principle.

• When the uncertainties in measurements of momentum p and position x for a particle are multiplied together, the product must be equal to or greater than Planck’s constant, h, divided by 2, which is represented as (called h-bar).

px




• The is “uncertainty in measurement of”: p is uncertainty in measurement of p and x the uncertainty in position. The product of uncertainties must be equal to or greater than () the size of .




• Applies to uncertainties of measurements of energy and time. The uncertainty in knowledge of energy, E, and the duration taken to measure the energy, t, are related by the

expression: Et .




• Heisenberg’s uncertainty principle applies only to quantum mechanics.

• It does not apply to– uncertainties of macroscopic laboratory

measurements.– a shield of nature’s secrets.– the notion that science is basically uncertain.


To which of these does Heisenberg’s uncertainty principle apply?

A. Measuring room temperature with a thermometer

B. Momentum and distances of a high-speed bullet

C. A public opinion survey


Uncertainty PrincipleCHECK YOUR ANSWER


To which of these does Heisenberg’s uncertainty principle apply?

A. Measuring room temperature with a thermometer

B. Momentum and distances of a high-speed bullet

C. A public opinion survey


Explanation:

Heisenberg’s uncertainty principle involves the unavoidable interaction between nature at the atomic level and the means by which we probe it.

Uncertainty PrincipleCHECK YOUR ANSWER


Complementarity

Complementarity• Wholeness often means accepting

alternate explanations for natural phenomena.

• Opposite ideas can complement one another (light can be both a wave and a particle).

• Bohr chose the yin-yang diagram to illustrate complementarity.

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© 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 31: LIGHT QUANTA