The nature of light
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Atoms
Atoms consist of a nucleus of protons (which have a
positive electrical charge) and neutrons (no charge),
surrounded by a cloud of electrons (negative charge)
Atoms
The type of element is determined the number of protons,
also called the atomic number. Sometimes atoms of the
same element can have different number of neurons;
these are called isotopes of that element.
Atoms
Atoms
The electrons can have different energy levels. What’s
really interesting is that the electrons can’t have just any
amount of energy, but can only have particular amounts
of energy (i.e. the energy levels are discrete, or quantized.
They are not continuous.)
Atoms
Electrons aren’t like particles that orbit the nucleus in the
way that planets orbit stars. Individual electrons really
are “smeared out” into clouds. But we often visualize
electron energy levels like this anyway:
Atoms
Electrons aren’t like particles that orbit the nucleus in the
way that planets orbit stars. Individual electrons really
are “smeared out” into clouds. But we often visualize
electron energy levels like this anyway:
n=1
(the ground state)
n=2
If an electron gains or loses the exact
n=3
right amount of energy, it can transition
n=4
between different energy levels.
n=5
Atoms
Energy level diagrams: different elements have different
electron energy levels
Atoms
Energy level diagrams: different elements have different
electron energy levels
ionization — this is when an
electron gets enough
energy to escape the atom
The nature of light
There are some key experiments that reveal the nature of
light
•
Diffracting light through a prism — reveals that
white light is made of many colors.
•
The double-slit experiment — shining
(monochromatic) light on a screen with two slits
produces strange patterns on the other side. Reveals
the wave nature of light.
•
The photoelectric effect — shining light on a surface
can free electrons, but only if the light has the right
color. Reveals the particle nature of light.
The nature of light
There are some key experiments that reveal the nature of
light
•
Diffracting light through a prism — reveals that
white light is made of many colors.
•
The double-slit experiment — shining
(monochromatic) light on a screen with two slits
produces strange patterns on the other side. Reveals
the wave nature of light.
•
The photoelectric effect — shining light on a surface
can free electrons, but only if the light has the right
color. Reveals the particle nature of light.
The diffraction of light
The diffraction of light
But Newton showed that if you
shine light of only a single color
it through a prism, it does not
diffract. This shows that the
different colors were present in
the light originally, and that they
are not created inside of the
prism
we look at a rainbow. Light’can come,in: an array of types
. A spectral curve (like the one:shown:.below) isa graph
given off by an object each second’versuSthe different
The diffraction
light
a specific
color of light onofthe
horizontal axis, the height
energy is being given off at’that particular wavelength..
for an
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orange !ight
than color. This is
You
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plotred
of and
brightness
versus
d end of the curve is higher than the, yioletend so the
called a spectrum.
n color.
atest
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Violet Indigo Blue Green Yellow Orange Red
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Figure 1
2) Imagine that the blue light and
orange light from the source were
blocked. What color(s) would now be
present in the spectrum of light observed?
The diffraction of light
___—
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3) Which of the following is the most
Violet Indigo Blue Green Yellow Orange
accurate
spectral
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for the
You
can
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plot
of brightness versus color. This is
spectrum described in Question 2?
Figure 1
called a spectrum.
a)
b)
c)
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C)
C)
VIBGYOR
C)
VIBGYOR
4) What colors of light are present in 3b above?
VIBGYOR
Light can be made up of some combination of many
different colors, or only one color.
I
R
Red
The nature of light
There are some key experiments that reveal the nature of
light
•
Diffracting light through a prism — reveals that
white light is made of many colors.
•
The double-slit experiment — shining
(monochromatic) light on a screen with two slits
produces strange patterns on the other side. Reveals
the wave nature of light.
•
The photoelectric effect — shining light on a surface
can free electrons, but only if the light has the right
color. Reveals the particle nature of light.
The double-slit experiment
If you shine light of a single color through two slits, it
makes a pattern on the other side
The double-slit experiment
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
The double-slit experiment
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
+
The double-slit experiment
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
+
=
The double-slit experiment
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
+
The double-slit experiment
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
+
=
The double-slit experiment
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
The double-slit experiment
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
Waves
A wave is something that transmits energy without
carrying material along with it
Waves
A wave is something that transmits energy without
carrying material along with it
wavelength — the distance
between adjacent peaks
frequency — the number of times
each second that a point moves
up and down
When wavelength goes up,
frequency goes down:
frequency = c/wavelength
Waves
The color of light is determined by it’s wavelength or
frequency.
• Redder light has higher wavelength and lower frequency
• Bluer light has lower wavelength and higher frequency
Waves
So a spectrum is really a plot of brightness versus
wavelength
Waves
So a spectrum is really a plot of brightness versus
wavelength
Waves
So a spectrum is really a plot of brightness versus
wavelength
Waves
The human eye is only sensitive to a narrow range in
wavelength or frequency. That’s why you can’t see light
with shorter wavelengths (like x-rays) or longer
wavelengths (like infrared and radio).
The thermal spectrum (aka “blackbody” spectrum)
Dense objects emit light with a certain spectrum depending
on their temperature. Objects with relatively cold
temperatures emit most of their light at long wavelengths
(like in the infrared). Objects with high enough
temperatures can emit light in at visible wavelengths.
The thermal spectrum (aka “blackbody” spectrum)
•
Midterm on Tuesday (scantron and fill-in-the-blank/
short answer)
•
No homework next week
Review
Atoms consist of a nucleus of protons and neutrons
surrounded by a cloud of electrons. The type of element is
determined the number of protons, also called the atomic
number. Sometimes atoms of an element can have
different numbers of neutrons; these are called isotopes.
Review
Electrons aren’t like particles that orbit the nucleus in the
way that planets orbit stars. Individual electrons really
are “smeared out” into clouds. But we often visualize
electron energy levels like this anyway:
n=1
(the ground state)
If an electron gains or loses the exactn=2
right amount of energy, it can transition
n=3
between different energy levels. But if it
n=4
gets enough energy then it can escape
the
atom entirely.
n=5
Review
There are some key experiments that reveal the nature of
light
•
Diffracting light through a prism — reveals that
white light is made of many colors.
•
The double-slit experiment — shining
(monochromatic) light on a screen with two slits
produces strange patterns on the other side. Reveals
the wave nature of light.
that displays the amount of energy given off by an object each second’versuSthe different
2) Imagine
that thecolor
blueoflight
wavelengths (or colors) of light.
For a specific
lightand
on the horizontal axis, the height
orange
light
from
the
source
a)
of the curve will indicate how much energy is being given offwere
particular wavelength.. b)
at’that
blocked.
What
color(s)
would
now
be
Figure 1 shows the spectral curve for an object emitting more red and orange !ight than
present
in the
spectrum
light 0observed?
0
indigo and violet. Notice that the
red end
of the
curve isofhigher
C) than the, yioletend so the
C)
I
I
I
object will appear slightly reddish in color.
___—
Review
V
of the following is the most
1) Which color of light has3)theWhich
greatest
energy output in Figure 1? accurate spectral curve for the
spectrum described in Question 2?
2) Imagine that the blue light and
orange light from the sourcea)were
blocked. What color(s) would now be
present in the spectrum of light
observed?
0
b)
0
VIBGYO
c)
4) What colors of light are present in 3b above?
0
0
C)
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B
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C)
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Violet Indigo Blue Green Yellow Orange Red
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Figure 1
What colors are resent in 3c above? Would this o
VIBGYOR
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Figure 1
VIBGYOR
V
a)
I
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Violet Indigo Blue Green Yellow Oran
___—
C)
3) Which of the following is the most
accurate spectral curve for the
spectrum described in Question 2?
B
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VIBGYOR
b)
VIBGYOR
c)
4) What colors of light are present in 3b above?
0
C)
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C)
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©r2QO8iearson Education, Inc.,
PubIishingas Pearson Addison-Wesley.
5
VIBGYOR
LE
What colors are resent in 3c above? Would this obect appear reddish or bluish?
VIBGYOR
VIBGYOR
4) What colors of light are present in 3b above?
Light can be made
up of some combination of many
different colors, or only one color. This is shown by
looking at it’s5 spectrum.
What colors are resent in 3c above? Would this obect appear reddish or bluish?
©r2QO8iearson Education, Inc.,
PubIishingas Pearson Addison-Wesley.
LECTURE-TUTORIALS FOR INTRODUCTORY A
SECON
Review
If you shine light of a single color through two slits, it makes
a pattern on the other side. This is because light behaves as a
wave, adding up constructively or destructively
Review
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
+
=
Review
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
+
=
Review
What’s going on? Light behaves like a wave. And when
you add two waves together, sometimes they add up
constructively, and sometimes they add destructively
Review
A wave is something that transmits energy without
carrying material along with it
wavelength — the distance
between adjacent peaks
frequency — the number of times
each second that a point moves
up and down
When wavelength goes up,
frequency goes down:
frequency = c/wavelength
Review
The human eye is only sensitive to a narrow range in
wavelength or frequency. That’s why you can’t see light
with shorter wavelengths (like x-rays) or longer
wavelengths (like infrared and radio).
Review
Dense objects emit a thermal (aka blackbody) spectrum
based on their temperature. Hotter objects are brighter at
all wavelengths, and peak and a shorter wavelength
The nature of light
There are some key experiments that reveal the nature of
light
•
Diffracting light through a prism — reveals that
white light is made of many colors.
•
The double-slit experiment — shining
(monochromatic) light on a screen with two slits
produces strange patterns on the other side. Reveals
the wave nature of light.
•
The photoelectric effect — shining light on a
surface can free electrons, but only if the light has
the right color. Reveals the particle nature of light.
The photoelectric effect
For centuries people argued over whether light is
continuous or whether it consists of a lot of individual
particles. Discovery of the wave nature of light suggested
that light is continuous. But this idea was challenged by an
experiment:
The photoelectric effect
•
In the 19th century physicists knew that light was a
wave, and that light carries energy.
•
So they thought that shining light onto a material
would give the electrons enough energy to become
ionized if the light was shining for long enough, or if
the light was bright enough.
•
But this turned out not to be the case! Experiments
showed that electrons are released only if the light has
high enough frequency. If the frequency is too low, it
does not matter how bright the light is nor how long it
shines on the material.
The photoelectric effect
So what does this mean? Light is
emitted or absorbed in discrete
packets. In other words, light
behaves like a particle. We call
these particles photons.
energy of a photon: E=hf
(where h is a constant
and f is the frequency)
The photoelectric effect
So an individual photon can free an individual electron
only if it has enough energy (i.e. a high enough frequency).
Otherwise, no matter how many photons you shine on a
material, the electrons will never absorb the photons.
Q: The higher energy of a photon,
A. the longer it’s wavelength
B. the shorter it’s wavelength
C. energy is independent of wavelength
Q: The higher energy of a photon,
A. the longer it’s wavelength
B. the shorter it’s wavelength
C. energy is independent of wavelength
The nature of light
We’ve established three very important things about light:
•
Light can be described by it’s spectrum (brightness
vs. wavelength)
•
Light behaves like a wave; it has a wavelength and a
frequency (l=c/f)
•
Light also behaves like a particle; it is absorbed and
emitted in individual photons (also sometimes called
wavepackets) with energy E=hf
How does light interact with matter?
There are three ways:
•
emission
•
absorption
•
transmission (i.e. light passes through the object)
•
reflection
How does light interact with matter?
Q: You see a rose in a garden. It is red. Which process
describes the light from the rose?
A. the rose emits red light
B. the rose absorbs red light
C. the rose transmits red light
D. the rose reflects red light
E. none of the above
Q: You see a rose in a garden. It is red. Which process
describes the light from the rose?
A. the rose emits red light
B. the rose absorbs red light
C. the rose transmits red light
D. the rose reflects red light
E. none of the above
A red object absorbs the light at most wavelengths but
it reflect the red light
Three different types of spectra
Continuous spectrum:
Emission line spectrum:
Absorption line spectrum:
Three different types of spectra
Three different types of spectra
A dense object will emit a continuous spectrum. This will
depend on it’s temperature, which is why it’s also called a
thermal spectrum (also a blackbody spectrum)
Three different types of spectra
A hot cloud of gas will emit an emission line spectrum.
Because of the thermal energy in the gas, some of the
electrons will get bumped up to higher energy levels. When
they move back down to lower energy levels, they will emit
photons corresponding to the difference in energy.
Three different types of spectra
If an electron is bumped up to
a higher energy level, it will
move back down very quickly,
releasing a photon
Three different types of spectra
If an electron is bumped up to
a higher energy level, it will
move back down very quickly,
releasing a photon
Three different types of spectra
A hot dense object illuminating a cool cloud of gas will
produce an absorption line spectrum. Most of the electrons
in the gas will be in the ground state. But photons from the
source that have exactly the right amount of energy will be
absorbed by the electrons, moving them into a higher
energy state.
Chemical fingerprints
Different elements have different electron energy levels.
So by studying looking at the wavelengths of the
emission/absorption lines, you can figure out what
element you’re looking at!
Chemical fingerprints
Different elements have different electron energy levels.
So by studying looking at the wavelengths of the
emission/absorption lines, you can figure out what
element you’re looking at!
•
05_MysteryGasComposition.htm
Example: the solar spectrum
The sun emits an absorption line spectrum, because it has
a hot core (thermal spectrum) with cooler gas on the
outside that produces absorption lines. These lines tell us
that the sun is mostly made of hydrogen and helium with
smaller amounts of heavier elements
Doppler shift
The Doppler shift refers to the change in wavelength and
frequency of a wave if the emitting object is moving
towards or away from you.
Doppler shift
The Doppler shift refers to the change in wavelength and
frequency of a wave if the emitting object is moving
towards or away from you.
The waves get bunched up if the object is moving
towards you, or stretched out if it is moving away
Doppler shift
The Doppler shift refers to the change in wavelength and
frequency of a wave if the emitting object is moving
towards or away from you.
Doppler shift
This happens for any type of wave, including sound
waves and light waves.
•
For an object moving toward you, the wavelength
goes down and the frequency goes up. In the case of
light, we call this a blueshift.
•
For an object moving away from you, the wavelength
goes up and the frequency goes down. We call this a
redshift.
Doppler shift
We generally measure the Doppler shift from shifts in the
wavelengths of spectral lines
Stationary
Moving away
Away faster
Moving toward
Toward faster
Doppler shift
We generally measure the Doppler shift from shifts in the
wavelengths of spectral lines
Stationary
Moving away
Redshifted
Away faster
Moving toward
Toward faster
Blueshifted
Doppler shift
•
By measuring the shift in the spectrum, we can estimate
very accurately the speed with which an object is
moving towards or away from us.
•
But this only gives us the speed along the line-of-sight
Large redshift
No shift at all, since it is moving
perpendicular to the line-of-sight
Small redshift, since it is moving
away from us but not very quickly