Physics 31 Questions

In the photoelectric effect, does brightness or frequency determine the kinetic energy of the ejected electrons? Which determines the number of the ejected electrons?

The frequency of the light affects the maximum energy of the ejected electrons. The brightness of the light is proportional to the rate at which electrons are ejected.

Does the photoelectric effect prove that light is made of particles? Do interference experiments prove that light is composed of waves? (Is there a distinction between what something is and how it behaves?)

The photoelectric effect proves conclusively that light has particle properties. Einstein attributed quantum properties to light itself, viewed radiation as a hail of particles. The number of photons in a light beam affects the brightness of the whole beam, whereas the frequency of the light controls the energy of each individual photon.

If an electron and a proton have the same de Broglie wavelength, this particle has the higher speed?

The same wavelength means that the two particles have the same momentum. This means that the less massive electron must travel faster than the heavier proton.

What is the uncertainty principle with respect to momentum and position?

The uncertainty in position times the uncertainty in momentum is greater than or equal to h-bar. Δp Δx = ћ

We don't notice the de Broglie wavelength for a pitched baseball. Is this because the wavelength is very large or because it is very small?

The wavelength of a pitched baseball is extremely small-- on the order of 10^20 times smaller than the atomic nucleus.

Which are more successful in dislodging electrons from a metal surface: photons of violet light or photons of red light? Why?

Violet light is more successful because the higher energy of a violet photon interacts with a single electron and gives it enough energy to escape the metal. Electron energy depends on frequency of the light. High frequency of (UV) waves ensures high energy per photon.

Which theory of light, the wave theory or the particle theory, did the findings of Young, Maxwell, and Hertz support?

Wave theory. Young: Double Slit experiment: wave phenomena. Maxwell: light carries energy in oscillating electric and magnetic fields. Hertz: sparking electric circuits to demonstrate electromagnetic waves (radio frequency)

Does light travel from one place to another in a wavelike or a particle-like way?

Wavelike. "Light travels as a wave and hits like a particle." A photon behaves as a particle when it is being emitted by an atom or absorbed by photographic film or detectors; behaves as a wave in traveling from a source to the place of detection.

Will brighter light eject more electrons from a photosensitive surface than dinner light of the same frequency?

Yes. The number of ejected electrons depends on the number of incident photons.

What is a quantum of light called?

a photon.

Does light behave primarily as a wave or as a particle when it interacts with the crystals of matter in photo- graphic film?

A particle. Many photons activating many grains produce the usual photographic exposure. The image is built up by individual photons that arrive independently. See how an exposure progresses photon by photon (fig. 31.5)

When does a photon behave like a wave? When does it behave like a particle?

A photon behaves as a particle when it is being emitted by an atom or absorbed by photographic film or detectors; behaves as a wave in traveling from a source to the place of detection.

In the formula E = hf, what does f stand for?

The frequency of electromagnetic oscillation. E = hf gives the smallest amount of energy that can be converted to light with frequency "f", with each photon in a stream throbbing at frequency "f" in the radiation of light

Does light interact with a detector in a wavelike or a particle-like way?

Particle-like. "Light travels as a wave and hits like a particle." A photon behaves as a particle when it is being emitted by an atom or absorbed by photographic film or detectors; behaves as a wave in traveling from a source to the place of detection.

What evidence can you cite for the wave nature of particles?

Particles exhibit two-slit interference. If electrons behaved only like particles, they would form two bands. Due to wave nature, they actually produce an interference pattern of fringes.

What is the principle of complementarity?

Quantum phenomena exhibit either wavelike or particle-like properties depending on the experiment conducted.

What does the term "quantum" mean?

Quantum: smallest elemental unit of a quantity. Radiant energy is composed of many quanta (photons). More photons in a beam of light = more energy in that beam.

Which has the lower energy quanta: red light or blue light? Radio waves or X-rays?

Red, radio. Relatively low frequency of waves ensures low energy per photon. UV photons deliver more energy than red to a molecule because the frequency of UV radiation is greater than red. X-rays, with even higher frequencies, can deliver even more.

What does it mean to say that something is quantized?

That it is composed of many quanta, discrete packets.

Cite evidence that the idea of opposites as components of a wholeness preceded Bohr's principle of complementarity.

The Chinese created the idea of yin-yang.

How much total energy is in a monochromatic beam composed of 'n' photons of frequency 'f'?

The energy in a monochromatic beam of light containing 'n' quanta is E = nhf (hf = energy, f = frequency, h = Planck's constant, E = energy, n = amount)

Did Max Planck consider the energy of vibrating atoms to be quantized? The energy of light itself?

The energy of the vibrating atoms only. Planck assumed that energy in matter is quantized but that radiant energy is continuous.

What is the distinction in this book between passively and actively observing an event?

Active observing requires interacting with the system being observed, whereas passive observing does not. Observing vs Probing. Viewing steam from a cup of coffee vs placing a thermometer in the cup!

Does the photoelectric effect support the wave theory of light? The particle theory of light?

Particle theory. The photoelectric effect proves conclusively that light has particle properties. Einstein attributed quantum properties to light itself, viewed radiation as a hail of particles. The number of photons in a light beam affects the brightness of the whole beam, whereas the frequency of the light controls the energy of each individual photon.

When electrons are diffracted through a double slit, do they hit the screen in a wavelike way or in a particle-like way? Is the pattern of hits wavelike or particle-like?

Particle-like, wavelike. Electrons, like photons, strike the screen as particles, but the pattern of arrival is wavelike.

Why won't a very bright beam of red light impart more energy to an ejected electron than a feeble beam of violet light?

Each electron receives energy from a single photon and violet photons have more energy than red photons. Ejected energy of electrons is unaffected by brightness. Depends on frequency. Relatively low frequency of (red) waves ensures low energy per photon.

Suppose that you lived in a hypothetical world in which you'd be knocked down by a single photon, in which matter would be so wavelike that it would be fuzzy and hard to grasp, and in which the uncertainty principle would impinge on simple measurements of position and speed in a laboratory, making results irreproducible. In such a world, how would Planck's constant compare with the accepted value?

It would be insignificant! At this high scale, Planck's constant would have to be a larger amount.

When does light behave as a wave? When does it behave as a particle?

Light is emitted and detected as a particle and travels as a wave.

What evidence can you cite for the wave nature of light? For the particle nature of light?

Light is emitted and detected as a particle and travels as a wave. "Light travels as a wave and hits like a particle."

Why do photographs in a book or magazine look grainy when magnified?

Magazine photographs are composed of individual dark spots resulting from photons of light interacting with electrons in grains of silver in film or in digital detectors.

Is Heisenberg's uncertainty principle applicable to the practical case of using a thermometer to measure the temperature of a glass of water?

No. Although we probably subject the temperature of water to a change by probing it, the uncertainty principle only applies in the atomic/subatomic level.

If a measurement shows a precise value for the energy radiated by an electron, can that measurement show a precise time for this event as well? Why?

No. Applies to energy and time. The uncertainty principle forbids precise measurements of both simultaneously. The more accurately determined the energy of a photon/electron/particle, the more uncertain the time during which it has that energy.

If measurements show a precise position for an electron, can those measurements show the precise momentum also? Why?

No. The uncertainty principle forbids precise measurements of both simultaneously. The sharper one of these quantities is, the less sharp the other is.

Will high-frequency light eject a greater number of electrons than low-frequency light?

Not necessarily. The energy (not the number) of ejected electrons depends on the frequency of the illuminating photons. A bright source of blue light, for example, may eject more electrons at a lower energy than a dim violet source.

In which of the following are quantum uncertainties significant: measuring simultaneously the speed and location of a baseball, a spitball, or an electron?

Only for the electron. Quantum uncertainties are significant only in the atomic and subatomic realms.