Transcript Chapter 6

Chapter 5
Light:
The Cosmic Messenger
Properties of Light
• Particle or wave?
• Light is electromagnetic energy in which the
electric and magnetic fields vibrate.
• Light behaves as both a particle and a wave.
• Light as a Particle: Photon- discrete bundle of
light energy.
• Light as a Wave: Electromagnetic Waves
Wave-Particle Duality
Light
Electron beam
Diffraction Fringes
Electron Interference Pattern
Ref: P.G. Hewitt, Conceptual Physics, 9th ed.©2002
Pearson Education
Stages of film exposure show photon by
photon formation of photograph.
Some definitions for waves
• Wavelength : the distance between adjacent
peaks of a wave.
– We measure electromagnetic wavelengths in
nanometers or angstroms.
• Frequency: the number of peaks per second,
that pass by a certain reference point.
– We measure frequency in Hertz (Hz)
Many Forms of Light
The Material World
• Matter can exist in different phases.
• Solid
• Liquid
• Gas
What is Matter?
• Today, we know that all ordinary matter is composed of
atoms.
• Each different type of atom corresponds to a different
chemical element.
• Atoms can form molecules which can then form a number
of different material substances.
• Some molecules consist of two or more atoms of the same
element.
O2 – molecular oxygen
O – atomic oxygen
H2 – molecular hydrogen
H – atomic hydrogen
Atomic Structure
• A small dense nucleus lies at the center of an
atom.
• The nucleus is made up of protons and
neutrons.
• The nucleus is surrounded by particles called
electrons.
• The properties of an atom depend primarily
on the amount of electrical charge.
(protons and electrons)
Terminology
• atomic number: The number of protons in
an atom.
• atomic mass: The combined number of
protons and neutrons in an atom.
• isotope: Sometimes, the same element can
have more than the usual number of neutrons.
We call this an isotope.
– A proton and a neutron form an isotope of
hydrogen called deuterium.
Summary of Atomic Structure
Ions and Ionization
• The loss of one or more electrons (electrons are
negatively charged) leaves the remaining atom
with a net positive charge.
• Such charged atoms are called ions.
• The process of stripping electrons from atoms is
called ionization.
• At high temperatures, the atoms of a hot gas can
become ionized, creating a plasma phase of
matter.
Light in Everyday Life
• Power – describes the rate of energy use.
• 1 Watt = 1 Joule/sec.
• Spectrum – The component colors of a light source when
viewed through something that creates a spectrum.
• Ex: Prism, diffraction grating.
• Emission – The transfer of light energy out of the object.
• Absorption – The transfer of light energy into the object.
A mirror reflects light along a path
determined by the angle of reflection.
Notice that the angle of reflection
equals the angle of incidence.
Light Scattering: The movie screen scatters a narrow beam of
light into many that reach all members of the audience
• Transmission – The passing of light through
a body. If light passes through a body, it is
said to be transparent to that light.
• Reflection – The return of light off of a
surface.
• Note: A body may be reflective and/or
transparent to only part of the incident light.
Energy in Atoms
• Atoms contain electric potential energy in the
distribution of their electrons around their
nuclei.
• Consider the hydrogen atom, which is the
simplest atom.
Ionization
When the atom
contains the smallest
amount of electric
potential energy, we
say that the atom is
in the ground state.
If the electron gains
energy, it becomes
“smeared out” over a
greater volume.
If the electron gains
enough energy, it
can escape the atom
completely and we
have an ionized
atom.
The Discovery of the Quantum World
• The most surprising aspect of atoms was the
discovery that only particular energy transitions
can occur for the electron.
• This was the beginning of Modern Physics
(1910 – 1935) and a theory of Quantum
Mechanics was developed.
I’m
Continuous!
Classical System
Any height is
possible
I’m
Quantized!
Quantum System
Only discrete
“Quantized” step
heights are possible.
Energy Levels for H atom
Blue Light
Red Light
Light and Matter
• Matter may absorb light or emit light.
• We can measure the amount of absorption
or emission by specifying the Intensity of
the light (I) in Watts/m2.
Proto-typical Spectrum
Absorption and Emission by Thin Gases
Various Energy
Level transitions
in Hydrogen
Emission line spectrum
Absorption line spectrum
Atomic Emission Line Spectra
Molecular Emission Line Spectrum
Molecular Hydrogen (H2)
Thermal Radiation
• Hotter objects emit more total radiation per
unit surface area.
• Hotter objects emit photons with higher
average energy.
Thermal Radiation
Thermal Radiators emit Radiation
according to the Planck Curve
• Max Planck gives the
first successful
explanation for the
characteristic curves
of thermal radiators.
• This ushers in the
Quantum Theory.
• (1858- 1947)
Representative Planck Curves
Kirkhhoff’s Laws
• Any “Complex” object produces thermal radiation
over a broad range of wavelengths.
• When thermal radiation passes through a thin
cloud of gas, the cloud leaves “fingerprints” that
may be either absorption lines or emission lines,
depending on its temperature
Kirkhhoff’s Laws
• If the background source is colder than the
cloud, or if there is no background source at
all, the spectrum is dominated by bright
emission lines produced by the cloud’s
atoms and molecules
• Summary of Kirkhhoff’s Laws – Next Slide
Reflected Light
• The color of an object that does not act as a
source of light depends on the light that it
reflects.
• A planet may reflect certain colors from the
sun and absorb others. The reflected light
gives the planet its characteristic color,
while the absorbed light heats the surface
and influences its surface temperature.
Putting It All Together- a typical spectrum
The Doppler Effect
• If an object that is emitting a wave moves toward
us (or we toward it), then the wavelengths are
shortened (frequency is increased).
• If an object that is emitting a wave moves away
from us (or we move away from it), then the
wavelengths are lengthened (frequency is
decreased).
• For light, the lengthening of the wavelength due to
this Doppler shift is called a Doppler Red Shift.
The shortening is called a Doppler Blue Shift.
This observer sees the light
red-shifted
This observer sees the
light blue-shifted
Doppler Shift
• The Doppler shift also appears in the
observed spectrum. (See next slide)
Collecting Light With A Telescope
• Light- Collecting Area: The cross-sectional area
of the primary mirror or objective lens.
• The Light collecting area determines how much
light the telescope can focus.
• Angular Resolution: The smallest angular
size a telescope can measure.
– Ex: The Hubble Space telescope has an angular
resolution of 0.05 arcseconds.
• In general, larger diameter telescopes have
both a greater light-collecting area and a
better angular resolution.
• Other factors that limit resolution include:
– The engineering and construction of the
telescope.
– Effects of the atmosphere for ground based
telescopes.
Basic Telescope
Design
1m refractor at
Yerkes
Observatory
5m Reflector at Mt. Palomar Observatory
Telescope Designs
10-m Keck Telescope
The
Hubble
Space
Telescope
Use of Telescopes
• Imaging: provides pictures of astronomical
objects.
• Spectroscopy: Involves dispersing the light
into a spectrum for analysis of composition,
temperature, relative motion, etc.
• Timing: Monitors how the light intensity
hitting a detector varies with time. This can
provide information about the objects
rotational motion.
Atmospheric Effects on Observations
• Light Pollution: The scattering of bright
artificial light from cities and towns at
night. This causes glare which reduces
“seeing” conditions.
• Turbulence: The changing direction of air
motion near the telescope bends the light in
shifting patterns. This causes the
“twinkling” of stars but also blurs
astronomical images.
Some Ways to Fix the Problems Due to The Atmosphere
• Adaptive Optics: The controlled
deformation of the primary mirror in order
to counteract the effects from atmospheric
distortion.
• Adaptive Optics can eliminate most
atmospheric distortion.
Ground based observation
of a double star
Same object now using
adaptive optics system
Telescopes Across The Spectrum
X-Ray
Telescopes
Radio Telescopes – 305m
radio telescope at Arecibo,
Puerto Rico.
The Trajectory of Voyager 2
Images of Jupiter from the
Mission
Voyager
Saturn (from Voyager Mission)
Neptune against Triton’s Horizon
From Voyager Mission
What lies ahead?
Artist’s Conception of Lunar Observatory