Transcript Light and Atoms

Introduction to Astronomy
• Announcements
– HW #1 due Wednesday 06/18/2008
– Course Reserves
Project Details
Project 1: Homemade
Spectroscope
• Chapter 4 in textbook
• In this project, you will build a simple
spectroscope from a cardboard tube,
aluminum foil, and a grating (which will be
supplied)
• Construction details can be found at the
end of Chapter 4 (pg. 144)
• You will sketch the spectra you see (more
on this later) from:
– Fluorescent light
– Mercury/Sodium vapor streetlight
– Ordinary incandescent light bulbs
– The blue sky (DO NOT LOOK AT THE SUN!)
– Flames (use gas-burning stove, add salt to
see sodium emission lines and copper wire to
see green copper emission lines)
– Extra credit for any other sources you want
• This writeup must include a picture of your
spectroscope. You will keep the real thing.
Project 2: Moon Observation
• Chapter 6 in textbook
• Look at the Moon on an evening when it is
nearly full. Make a sketch of the light and dark
markings that you see on its surface with the
naked eye.
• Then observe the Moon with binoculars or
through a telescope (PDO is helpful here) and
make an enlarged sketch that shows more
detail. Mark & identify a few of the craters you
can see.
• Estimate the diameter of these craters from the
knowledge that the Moon’s radius is about 1000
miles (1700 km). How big is the largest crater
you can see compared to the size of Logan?
Can you you see any lunar rays? If so, sketch
them on your drawing. How long are the rays?
• Can you mark the landing sites where humans
have touched-down?
• SHOW ALL STEPS OF YOUR WORK!!!
Project 3: Solar Observation
• Chapter 11 in textbook
• NEVER LOOK DIRECTLY AT THE SUN WITH
THE NAKED EYE, OR THROUGH
BINOCULARS/TELESCOPE!!!!!!!!!
• Measure the diameter of the Sun.
– Take a piece of thin, dark cardboard and put a small
hole in it. Hold it about 1 meter (3 feet) from a piece
of white paper so that a small image of the sun
appears on the paper.
– Carefully measure the distance (d) between the
cardboard and the piece of paper and the size of the
Sun’s image (s) on the paper.
– On a separate piece of paper, draw two straight lines
that cross with a small angle between them (see
figure)
• Draw two small circles between the lines
as shown in the figure. Convince yourself
that if D is the distance to the sun (1 AU),
and S is the Sun’s diameter, then S/D =
s/d
– s = size of Sun’s image
– d = distance between paper and cardboard
• Look up the value of D, then solve for S
– Does it agree with the value in table 11.1?
• SHOW ALL YOUR WORK!!!
Light & Atoms
Light & Atoms
What is Newton holding?
What were the results of
this experiment?
Properties of Light
• Wave-particle duality
“Light is a wave on Monday, Wednesday, and Friday, and a particle
on Tuesday, Thursday, and Saturday. On Sunday, we have to think
about it… “
– Light has wave-like properties, and particlelike properties, depending on the type of
observation…
– Weird, right?
– Analogy: you are wearing a hat. Two people
observe you from different positions, but only
the one wearing glasses sees the hat…
• Wave-like
– Interference, diffraction
– Like overlapping
water waves…
• Particle-like
– Photoelectric effect
– Like a game of marbles…
The Schizophrenic Photon
• Interference cannot be described by the
particle model, and the photoelectric effect
cannot be explained by the wave model
– But we have observed both!
Similarities
This “sine”-wave goes on
forever in both directions,
so it is hard to pinpoint
the exact “location” of the
wave…
Is the energy this wave carries here?
A particle, on the other hand,
is very localized, so it has a
well-defined position…
or here?
or here?
But adding many
different waves
gives a very
localized
“wave-packet”…
…and these wavepackets behave a
lot like particles!
Quantum Mechanics calls this
the “wavefunction” of the particle,
and describes the likelihood that
the particle can be found at various
positions.
Less likely (but still possible)
to be here
More likely to be here
We will usually use the wave
model of light from here on
out…
…but we’ll briefly revisit the
photon model when we talk
about CCDs in the next
chapter
Properties of Light
• Color
– Not physical, all a psychological construct to
help the brain sort out different wavelengths
of visible light English physicist John Dalton (1766-1844),
– λred = 700 nm
– λblue = 400 nm
– 1 nm = 10-9 m
worked on colored shadows, color blindness
when he discovered pink flowers appear blue
to him…
He became obsessed with trying to discover
the cause of color-blindness, so he arranged
for his doctor to REMOVE ONE OF HIS EYES,
so Dalton himself could dissect it to look for
blue fluid inside that would cause his condition!
Characterizing Light as a Wave
• Self-sustaining electric and magnetic
vibrations
Characterizing Light as a Wave
• Wavelength
– Distance between successive “crests” or
“troughs” of the wave
Characterizing Light as a Wave
• Frequency
– Imagine you are standing next to a traveling light
wave (or water wave, if you prefer) that passes you…
– How many peaks pass you in 1 second?
– Frequency of light =
Speed of Light = c
Wavelength
λ
– Speed of Light, c = 3.0 x 108 m/s
Properties of Light
• “White” light
– Mixture of all visible colors
– Why doesn’t mixing paint of all colors produce
white paint?
• Chemical reactions due to pigment
The Visible Spectrum
Our eyes are sensitive only to an EXTREMELY narrow range of light waves
 “Visible” or “Optical” light
The Electromagnetic Spectrum
• Visible light constitutes a tiny, tiny fraction
of the whole range of light
• Our eyes are only sensitive to visible light,
but other types of “light” are all around us…
– Radio waves, TV waves, cellphone signals,
body heat…
– What would the world be like if you could see
at radio wavelengths?
The EM Spectrum on Earth
• Radio
– Music, television programs encoded into longwavelength waves
– Wireless & bluetooth devices
– Communications
• Infrared (IR)
– Distinguish between hot and cool objects
– Heat lamps at fast-food places & cafeterias
– Nerves in skin register this type of light as heat
• Visible
– Everything we can physically see
– Light bulbs
– Reflected sunlight (on Earth)
– Color
• Ultraviolet (UV)
– Suntanning
• Skin cells containing melanin produce Vitamin D
when they absorb UV light
– Snow blindness
– Blacklights & security “watermarks”
• X-Ray
– Medical, dental X-rays
– Dock scanning equipment
– By-product of atomic/nuclear detonation
• Gamma-Ray (γ)
– Highest energy
– Dock scanning equipment
– Radiation pasteurization
• Some normal perishables (meat, milk, fruits & vegetables, etc)
can be kept fresh (unrefrigerated) for weeks with a healthy
dose of radiation to kill off anything nasty.
– Atomic/nuclear weaponry
The EM Spectrum in Space
• Radio:
Pulsars, star remnants
• Microwaves: Cold interstellar clouds,
cosmic background radiation
• IR:
Young stars, planets, dust
• Visible:
Stars, the sun
• UV:
Hot, bright stars
• X-Ray:
Collapsed stars, black holes
• γ-Ray:
Active galaxies, GRBs
The EM Spectrum
• All these different types of light are the
SAME phenomena
– Self-sustaining vibrations of electric and
magnetic energy
• The shape of these vibrating energies determines
if the light is IR, UV, visible, etc…
• Energy carried by light wave of
wavelength, λ:
– Energy = hc / λ
• Which carries more energy?
– Red light or Blue light ?
– Blue light or X-rays ?
– Infrared light or radio waves ?
– Gamma rays or Ultraviolet waves ?
Properties of Light
• Temperature
– Hot objects emit light (electric stove, an iron
worked by a blacksmith)
– Hotter objects emit shorter-wavelength light
• Wien’s Law (pronounced ‘Veen’)
–
–
–
–
Cool stove, black element
A little hotter, red element
A little hotter, yellow element
Very hot, white element
• Wien’s Law
– Temperature = constant
λmax
– One of most important tools for astronomers
to measure temperature of stars,
planets, galaxies, etc…
Wien’s Law
• Example
– Someone states that because an apple looks
red, it must be emitting red light. Fortunately,
you have taken USU 1040 and know that
person is full of it. How would you show
them?
– We can assume the wavelength of the red
light is ~ 700 nm
– Using Wien’s Law, we can calculate the
temperature that the apple must have in order
to emit mostly red light…
• We get Temperature = 7000 °F
!!!
• Therefore, the apple clearly doesn’t EMIT the red
light, so it must only REFLECT it.
The Atom
The Atom
• Protons, Neutrons, and Electrons
• “Planetary” model of the atom
– Negatively-charged electrons orbit positively-charged
nucleus
– Electromagnetic force holds atom together
– Typical size ~ 10-10 m = 1 ten-billionth of a meter
• About X times smaller than the width of a human hair
– X ~ 500,000
• Planetary model is easy way to visualize
atoms
– But it is ultimately wrong!
– Accelerated charges radiate photons (light
energy)
• Therefore, an orbiting electron would constantly
lose energy (accelerated by centripetal force) and
move to progressively lower orbits
– Imagine the International Space Station
in orbit…
• Ultimately, it would spiral in to the nucleus and the
atom would destroy itself.
• Why is this clearly incorrect?
Quantized Atoms
• Electrons only allowed to orbit at certain,
discrete distances
– Painter on scaffold
– Developed from theory that even electrons
have wave-like properties (like light)
• “matter waves”
• ONLY at small scales
– a person walking through a door does not diffract (spread
out) into multiple people.
– Ice cubes do not suddenly teleport out of your glass and
into your pocket
A fundamental principle of Quantum
Mechanics:
The electron does not orbit the nucleus.
It can be anywhere in the
electron “cloud”, but we can’t know
precisely where until we measure it
Origin of Light & Spectra
• Electrons are not confined to single orbits.
• They can move to higher or lower orbits with
different energies, under the right
circumstances.
• Spring analogy
– Imagine proton and electron are connected by a
spring.
• To move them further apart, must supply energy to stretch
spring
• To move them closer together, some energy from stretched
spring is released as the spring de-stretches
• Analogy
– Fast lane & slow lane highway
• Merging into fast lane REQUIRES energy
• Merging into slow lane GIVES UP energy
– Same for electrons jumping from one orbit to
another
• Defines EMISSION & ABSORPTION of light….
• Emission of light energy = de-stretching the
spring
• Absorption of light energy = stretching the spring
• Conservation of Energy
– Rules the Universe, you will NEVER
break this law.
– Energy of emitted light = difference in
energy between upper and lower levels
– Difference between energy of upper & lower level =
energy of absorbed light ( if NOT equal, NO
absorption occurs)
HOW is light emitted?
• The positively-charged nucleus and the
negatively-charged electrons form a
system with some amount of “stored”
electrical energy
– Like a battery, positive and negative terminals
• If an electron moves to a lower orbit,
closer to the nucleus, it creates an
electrical disturbance in the system
• A fundamental principle of
electromagnetism is that an electric
disturbance creates a magnetic
disturbance, and vice versa
– Maxwell’s Equations
• The electrical disturbance produced by the
electron moving down to a lower orbit
creates a magnetic disturbance, which
creates an electric disturbance, which
creates a magnetic disturbance, ad
infinitum
– Viola! A self-sustaining vibration of electric
and magnetic energy = Light !
Use in Astronomy
• Because we cannot directly measure
astronomical sources (with a probe, e.g.),
we must analyze the light we get from
them
– “Spectroscopy”
– Because the light we receive comes from the
very hot atoms in a star, we expect that some
properties of the light can tell us about what
atom(s) emitted or absorbed it…
• Yes, we can tell a whole lot just from light!
Emission Spectra
• Produced when electrons move from
higher energy orbits to lower energy orbits
– Emitting light in the process
• Because only certain orbits are allowed,
only certain transitions are allowed,
therefore only certain wavelengths of light
are observed.
• Different atoms have different sets of
allowed electron orbits, so different atoms
produce different emission spectra.
– Not too long ago, it was thought that all atoms
emitted the same light…triumph of quantum
mechanics that it was able to describe the
different spectra observed…
Absorption Spectra
• Now, suppose we shine a light through a
cloud of Hydrogen gas
– The light that matches the energy difference
between the upper and lower levels of a
Hydrogen atom will be absorbed by that atom,
while other wavelengths will pass unaffected.
– This causes the spectrum to contain all
normal colors, but with dark bands at the
absorbed wavelengths
• “Absorption spectrum”
• Absorption lines appear at the same
wavelengths as emission lines, for a given
element.
• Emission spectra tell us about how hot an
object is, and what it is made of.
• Absorption spectra tell us about what lies
between us and an object.
Announcements
• Homework #1 due tomorrow
• First Project Due 30 June 2008 (Monday)
• Class Website Troubles…
Radio spectrum of cold
gas cloud
X-ray spectrum of hot gas
From exploding star
Stellar Classification by Spectra
The Doppler Shift
• Can determine chemical composition of
object by emission and absorption spectra,
but how?
– Compare observed lines with pure lines
measured in laboratory
• “line catalog”
– But any motion of the object will change the
observed wavelengths of emission and
absorption lines:
• Analogy
– Firetruck approaches  high pitch
• Sound waves “pile up” in front of firetruck, moving
toward you…
– Firetruck recedes  low pitch
• Sound waves “stretch out” behind firetruck, moving
away from you…
• Exact same thing can happen with light
waves
• If atom moves toward you when it emits
light:
– Wavelength decreases: “blueshifted”
• If atom moves away from you when it
emits light:
– Wavelength increases: “redshifted”
Doppler Shift
• Physics can get you in trouble with the
law…
– Photoradar speed-traps use the Doppler
effect to measure car speeds
Atmospheric Absorption
• Gases in Earth’s atmosphere (N2, O2, Ar, CO2)
absorb light from distant sources
– Sunlight
– Astronomical sources
• “Atmospheric Window”
– The reason our eyes are sensitive to visible light is
because it is NOT easily absorbed by the Earth’s
atmosphere
– This is also the reason why we need space-based
telescopes to observe in the IR, UV, X-ray regions of
the EM spectrum
NEXT TIME
• Telescopes
– How do we capture all these light waves?