Transcript Chapter-29 Particles and Waves

Chapter-29
Particles and Waves
This is the central portion of the Eagle
Nebula located about 7000 light-years
from earth, as seen by the Hubble Space
Telescope. Among the topics in this
chapter are particle-like entities called
photons. As we will see, they play the
central role in the process of photoevaporation, which allows astronomers
to peer into the dense star-forming
regions of the nebula.
A newborn star can be seen on the
surface of an EGG, evaporating gaseous
globule.
Electron Diffraction
The ability to exhibit interference effects is an essential
characteristic of waves.
One of the most incredible discoveries of twentiethcentury physics is that particles can also behave like
waves and exhibit interference effects.
(a) If electrons behaved as discrete particles with no
wave properties, they would pass through one or the
other of the two slits and strike the screen, causing it to
glow and produce exact images of the slits.
(b) In reality, the screen reveals a pattern of bright and
dark fringes, similar to the pattern produced when a
beam of light is used and interference occurs between
the light waves coming from each slit.
The electron exhibits a dual nature, with both particlelike characteristics and wave-like characteristics.
The Wave-Particle Duality
Scientists now accept the wave-particle duality as an essential part
of nature:
Waves can exhibit particle-like characteristics, and particles
can exhibit wave-like characteristics.
29.2. Blackbody Radiation and
Planck's Constant
The electromagnetic radiation emitted by a
perfect blackbody has an intensity per unit
wavelength that varies from wavelength to
wavelength, as each curve indicates. At the
higher temperature, the intensity per unit
wavelength is greater, and the maximum occurs
at a shorter wavelength.
Photons and Particles
Photoelectric Effect
Evidence for particle (photon) nature of light
comes from a phenomenon called the
photoelectric effect, in which electrons are
emitted from a metal surface when light
shines on it.
In the photoelectric effect, light with a
sufficiently high frequency ejects electrons
from a metal surface. These photoelectrons,
as they are called, are drawn to the positive
collector, thus producing a current.
Energy of a Photon
In 1905 Einstein presented an explanation of the photoelectric effect that took
advantage of Planck’s work concerning blackbody radiation. It was primarily
for his theory of the photoelectric effect that he was awarded the Nobel Prize
in physics in 1921. In his photoelectric theory, Einstein proposed that light of
frequency f could be regarded as a collection of discrete packets of energy
(photons), each packet containing an amount of energy E given by: (where h is
Planck’s constant)
Digital Camera
Digital cameras use an array of charge-coupled devices instead
of film to capture an image.
Visible Light CCD
A CCD array consists of a sandwich of semiconducting
silicon, insulating silicon dioxide, and a number of
electrodes. The array is divided into many small sections or
pixels, sixteen of which are shown in the drawing. Each
pixel captures a small part of a picture. Digital cameras for
consumers (rather than professionals) have between one
and five million pixels, depending on price. The greater the
number of pixels, the better is the resolution of the
photograph. The blow-up shows a single pixel. Incident
photons of visible light strike the silicon and generate
electrons via the photoelectric effect. The range of energies
of the visible photons is such that one electron is released
when a photon interacts with a silicon atom. The electrons
are trapped within a pixel because of a positive voltage
applied to the electrodes beneath the insulating layer. Thus,
the number of electrons that are released and trapped is
proportional to the number of photons striking the pixel. In
this fashion, each pixel in the CCD array accumulates an
accurate representation of the light intensity at that point on
the image.
Automatic garage door openers
Automatic garage door openers have a safety feature that prevents the door from
closing when it encounters an obstruction.
A sending unit transmits an invisible (infrared) beam across the opening of the door.
The beam is detected by a receiving unit that contains a photodiode.
When infrared photons strike the photodiode, electrons bound to the atoms absorb
the photons and become liberated. These liberated, mobile electrons cause the
current in the photodiode to increase.
When a person walks through the beam, the light is momentarily blocked from
reaching the receiving unit, and the current in the photodiode decreases. The
change in current is sensed by electronic circuitry that immediately stops the
downward motion of the door and then causes it to rise up.
Eagle Nebula
A giant star-forming region some 7000 light-years from earth.
These clouds extend more than a light-year from base to tip and are the
birthplace of stars.
A star begins to form within a cloud when the gravitational force pulls together
sufficient gas to create a high-density “ball.”
The process of photoevaporation allows astronomers to see many of the highdensity regions where stars are being formed.
Photoevaporation is the process in which high-energy, ultraviolet (UV) photons
from hot stars outside the cloud heat it up. As photoevaporation proceeds,
globules of gas that are denser than their surroundings are exposed. The
globules are known as evaporating gaseous globules (EGGs), and they are
slightly larger than our solar system.
THE MOMENTUM OF A PHOTON
AND THE COMPTON EFFECT
The phenomenon in which an X-ray photon is scattered from an electron, with
the scattered photon having a smaller frequency than the incident photon, is
called the Compton effect.
Compton Effect
Compton showed that the difference between the wavelength λ’
of the scattered photon and the wavelength λ of the incident
photon is related to the scattering angle θ by:
THE DE BROGLIE WAVELENGTH
AND THE WAVE NATURE OF
MATTER
This photograph shows a highly magnified view of a Drosophila fruit fly,
made with a scanning electron microscope. This microscope uses electrons
instead of light. The resolution of the fine detail is exceptional because the
wavelength of an electron can be made much smaller than that of visible
light. (David Scharf/SPL/Photo Researchers, Inc.)
The de Broglie wavelength
where h is Planck’s constant and p is the magnitude of the relativistic momentum
of the particle.
The De Broglie Wavelength of an
Electron and of a Baseball
Determine the de Broglie wavelength for (a) an electron (mass = 9.1x 10-31
kg) moving at a speed of 6.0x 106 m/s and (b) a baseball (mass = 0.15 kg)
moving at a speed of 13 m/s.
Electron version of
Young’s double-slit
experiment