*A: Any hot, dense object will glow in some range of the electromagnetic
spectrum. In other words, hot, dense objects approximate black
bodies. In
sun's case, the original source of its light are the gamma rays released
by the nuclear reactions in the core. Recall that gamma rays
are photons
with much more energy than photons which fall in the range that we
can see.
However, as gamma rays make their trek from the core to the surface
of the
sun, they get very bonked around by all the dense particles in the
sun. A
photon from the core can't travel very far at all before encountering
an
electron here or a proton there. Interactions with these particles
cause
the high energy photon to get degraded into more than one, lower-energy
photons (the details of this process are too complicated to get into
here).
So by the time the energy reaches the surface, instead of a few really
energetic gamma rays, we have a crapload of moderately-energetic photons,
whose distribution peaks in the visual part of the EM spectrum.
Q: Will an element with more energy levels (bigger atom) show more lines
on
the emission-line spectra than an element with less energy levels?
(because
electron jumps occur more rapidly with more energy levels?)
A: I am not actually certain that there is a correlation between the
size
of an atom and the number of energy levels available to its electrons.
If
one atom DID have more energy levels available to it, there would be
more
emission lines, since there would be more possible jumps for an electron
to make from one level to another. Also, jumps occur pretty much
instantaneously, and for all purposes, the electron NEVER EXISTS BETWEEN
ENERGY LEVELS! Does this make sense? Not according to our
intuitive
understanding of the macroscopic world. But when you get down
to the
microscopic world, the laws of quantum mechanics do weird things which
cannot readily be explained other than by saying, "That's just how
nature
works."
Q: Why do some atoms absorb light without releasing it? (and thus form
the
absorption-line spectra) And why do some atoms absorb light and then
release
it almost at the same time, thus produce an emission-line spectra?
what
determine this?
A: Imagine a beam of white light (in other words light containing a
continuous
rainbow of colors) heading RIGHT AT YOU. Then put a gas in the
path.
The atoms and molecules in the gas will absorb the very specific wavelengths
of light which correspond to differences in energy levels of their
electrons.
Light not at these wavelengths will continue through the gas unmolested,
and
still head right toward you. Now atoms don't like to stay excited,
so they
will probably have their electrons drop back down to the lower energy
states,
thus releasing photons of the same wavelengths they earlier absorbed.
However, when the photons go flying off, they will do so IN A RANDOM
DIRECTION,
so light that was once headed right at you is now heading somewhere
else,
hence the dark absorption lines.
Q: Can you explain more about the types of spectra: continuous
spectrum, emission-line spectrum, and absorption-line spectrum.
A: A hot, dense object emits a continuous (blackbody) spectrum.
A hot,
diffuse
gas will emit an emission spectrum (in other words, the only light
that
comes
from a hot, diffuse gas corresponds to the differences in the energy
levels
of the atoms and molecules in the gas. An absorption spectrum
is formed
when a beam of continuous light is intercepted by gas as described
above.
Q: Why is the ozone layer hotter than the atmosphere above and below it?
*The ozone layer is hot for the same reason that it is useful to us
and our
health. Ozone is a molecule containing three oxygen atoms.
This particular
molecule is very good at absorbing ultraviolet radiation (often called
UV
light). The sun radiates quite a bit UV light, which is harmful
to us
humans. (It can cause skin cancer, for example.) But the
ozone in the
atmosphere can absorb the UV photons, thus shielding us from them?
So
where does the energy from the UV photons go? It heats up the
ozone layer.
Q: I still don't understand how light can have both wave-like and
particle-like properties.
A: In certain circumstances, in order to explain the results of an
experiment, you must accept that light has properties of waves.
For
instance
it can form interference patterns when passed through slits.
In other
experiments, it behaves like particles. For example, you can
count exactly
how many photons excited electrons in your detector.
It's weird to say that light is both a particle and a wave, but that
is
simply how nature designed light.
Q: Could you explain how to use the Wien's law?
A: Wien's Law allows us to calculate the temperature of a hot, glowing
object by looking at which wavelength of light is radiated more by
that
object than any other wavelength. For example, the sun radiates
more
light in yellow than in any other color or wavelength. The wavelength
of yellow light is about 500 nm. Plug into Wien's law:
T = (3x10^6 K nm)/wavelength = (3x10^6 K nm)/500 nm. The nm cancel
and you
find that the surface temperature of the sun is about 6000 K.
Q: Also can you explain the equation for the doppler effect
The equation for the doppler effect is:
(lambda - lambda-naught)/(lambda-naught) = v/c
where lambda = the observed wavelength
lambda-naught = the emitted wavelength
v = the speed of the wave-emitting object
relative to the observer
c = the speed of the light (in the case
that we're dealing with
electromagnetic waves)
If you do a little algebra to this equation, you get
lambda = lambda-naught + lambda-naught x (v/c)
This equation says that the wavelength we observe (the left-hand side
of the
equation) = the emitted wavelength (first term on the right-hand side)
plus
a correction term proportional to the velocity of the object (the second
term on the right). Notice that if v=0, then the wavelength that
gets
observed is the same as the wavelength that gets emitted, which makes
sense.
Q: Would light from outerspace be more crisp, less fuzzy, if there was
no
atmosphere?
Yes. All the little particles of air and water vapor in our atmosphere
blur light coming in from space. If you looked through a powerful
telescope
in space at a distant star, you'd see a pretty crisp pinprick of light,
but
looking through a telescope on Earth you'd see kind of a bright blob.
(Now
you wouldn't see an absolutely crisp image in space, because the crispness
of
the image you get when looking through a telescope is still dependent
on the
resolution of the telescope, but it'd be A LOT crisper than you'd see
from
Earth.)
Q: Why are gases not usually black bodies?
A: *Gases are typically not dense enough. The sun, which is a
gas, is much
more dense than, say, the air in our atmosphere is. When a photon
of light
is
traveling through a dense gas, like the sun, it gets bonked around
a lot,
and winds up getting degraded into more than one, less-energetic photons.
(See my description above). In typical situations, gas isn't
very dense, so
when photons get emitted from it, they can escape relatively unmolested.
Q: Why do certain materials only absorb certain colors of light? Does
it
have
to do with density or tempuratire?
A: Atoms and molecules and ions only absorb photons of very specific
energies.
The energy of the absorbed photon must be EXACTLY the right amount
to
excite an electron to a higher energy level in the atom or molecule
or ion.
Q: From Chapter 3, I was wondering what kind of things are considered
blackbodies? What are some examples that are categorized as blackbodies?
A: Most any hot, dense object acts somewhat like a blackbody.
The burner
of an electric stove is a good example. When it's turned off,
it's color is
black -- it's reflecting no light. When you turn it on to low,
it glows a
dull red. When you turn it on to high, it glows a brighter orange.
You
can observe from this that when the temperature of a blackbody increases,
it's color becomes more blue (orange is closer to blue than red is)
and it
becomes brighter.
Q: What's the difference between objects "reflecting"
light and "emitting" light? Given objects in a room
are the same room temperature and obey Wien's law, are
they emitting any light?
A: If an object is said to be 'emitting' light, that means that the
object
is
glowing of its own accord. Consider a lit neon sign and a burner
on an
electric stove which is turned to 'high.' Both of these objects
glow of
their
own accord. If either of these things were in a room with no
windows and
all the lights were turned off, we would still see them glowing.
On the other hand, I'm wearing a red t-shirt today. The wavelength
of red
light is about 700 nm. According to Wien's law, something peaking
at
700 nm has a temperature of about 4000 K (which is 7000 degrees Fahrenheit).
Now, I don't feel that hot. Also, when I turn off the lights,
I can't see
my
t-shirt anymore. It is not glowing of its own accord. The
reason that I
can
see it and it appears red when the lights are on is that the t-shirt
is NOT
a
perfect black body, and that due to the chemical nature of the dyes
in the
fibers of my shirt, it reflects the color red more than any other color.
Bottom line: an object that is seen by emitting light glows in the dark;
an object seen by reflected light does not.
Q: In the equation for the Doppler shift, how do you find
lamda-knot? I know it stands for the emitted wavelength, the part we
haven't
measured, but how do we plug that in then when solving for the wavelength
shift?
A: Lambda-naught = emitted wavelength. Consider the H-alpha photon.
This
wavelength is emitted when an electron in a hydrogen atom drops down
from
the 2nd excited state to the 1st excited state. So how do we
know what
this wavelength is? We can get a jar of hydrogen gas, heat it
up in our lab
until it's glowing, then use a prism to spread out the light into a
spectrum
and measure the wavelengths of the emitted lines. We then can
compare this
wavelength to the wavelength of the H-alpha lines we see in stars to
determine what the velocities of those stars are.
Q: I'm confused as to what exactly is the difference between brightness
and
intensity?
A: These two terms can be used pretty interchangeably. Two terms
which are
important to keep separate are: brightness and luminosity. Luminosity
refers to the amount of energy per second leaving a star. You
can measure
it in Watts, just like a lightbulb. It is an intrinsic quantity
of the
star.
Brightness on the other hand is NOT intrinsic -- it depends on the
position
of the observer. The farther the observer is from a star of a
given
luminosity, the less bright it appears. Similarly, the farther
you are
from,
say a 100W light bulb, the dimmer it appears.
Q: I was just curious about the fact that it is said that the light
that is
emitted from the sun is of the temperature and of the composition that
it
emits a relatively yellow or orange light...but do the other planets
emit
any
visible light of their own? Is the light that we see reflected
from the
other planets all a reflection of the suns light, and if so how does
it get
it's differences in color? (why would neptune be blue as apposed
to the
marbled oranges of Jupiter aside from atmospheric differences would
there be
any other electromagnetic reason for their differences in color?)
*A: None of the planets radiates in the visible part of the electromagnetic
spectrum, although Jupiter radiates quite a bit in the infrared.
The reason
that different planets are different colors is that different chemicals
reflect different colors differently. For example, ferrous oxide
(rust)
reflects redish light better than any other color. Thus Mars,
which is
covered with the stuff, appears red when sunlight is reflecting off
of it.
Methane and ammonia reflect the color blue best, hence the color of
Neptune.
Q: One aspect of chapter three that confused me is why we can only see
a
small
portion of the light scale.
*A: While we only see a small portion of the light scale, it is the
portion
that the sun emits the most. Evolution and natural selection
have favored
lifeforms which can see the best in the type of light which is the
brightest.
Lifeforms which could see, say, in the infrared but not the visual
range
couldn't see as well because the sun isn't as bright in the infrared
as it
is in the visible part of the spectrum.
Q: First in Chapter 3 I did not quite understand how a spectrum is
formed. I didn't understand the heating process and the shifting
of
electron orbits. Please explain more on that.
A: When an electron drops (or de-excites) from a high energy state to
a
low energy state, the atom loses a VERY specific amount of energy.
A photon
with a wavelength corresponding to that energy is emitted. Similarly,
if
an atom starts of in a low-energy state, and a photon of EXACTLY the
right
energy comes along, that photon gets absorbed and the electron jumps
back
up the higher energy state. After a while the electron will drop
back down
and the photon will be released.
Q: pg 94 "Different colors travel at different speeds"
Is this through empty space or through a medium?
A: Through a medium only. Through space (which is a vacuum) all
wavelengths
travel at the same speed.
Q: pg 103 all elements have protons and the number of electrons are
always
equal to the number of protons?
*A: The atoms of all elements have protons. In fact, it is the
number of
protons which determines which element it is. Hydrogen has one
proton,
helium has two, etc. Also, all atoms have the same number of
electrons
as protons, so as to keep the atom electrically neutral. If an
atom loses
electrons, it has more positive protons than negively charged electrons,
and
it becomes what is called a positive ion. Similarly if an atom
gains more
electrons than protons, you get a negative ion.
Q: The question I have for chapter three has to do with
Figure 3.6. At a specific wavelength, a specific
color appears. But I still don't understand why there
are peaks on the curves. I would assume that as the
wavelength increases, the color would change to a red
and there would be only one curve to indicate this
change. Will you please clarify?
A: The three curves in fig. 3.6 correspond the light emitted by three
different objects of different temperatures. All three emit light
over a
certain range of colors, but the hottest one emits more blue light
than any
other color. Another, cooler object emits more yellow light than
any other
color. And the coolest object emits more red light than any other
color.
Q: The book describes photons as "particles." What exactly is
a "particle?"
Does it have protons, neutrons, and electrons?
A: 'Particle' in this case just means 'little discrete chunk.'
Saying that
photons behave like particles means that you can do things like count
them.
Q: So far in the class, it seems as though must things revolve or move
in
circles because of gravity, and objects not effected by a gravitational
pull
are said to move in a straight line. So how do waves come in/fit
in?
*A: Light, and all other waves, does travel in a straight line.
Later in
the
course we may talk about gravity bending the path that light travels.
This
is not because the light is attracted to the massive object like with
a gravitational force, but because (according to Einstein's general
relativity)
gravity actually bends space and makes IT curve, and the light just
travels in a straight line through curved space. Pretty wild,
huh?
Q: Why does the book choose waves?
A: In some circumstances, you need to consider the wave aspect of light
to explain how it behaves; in other circumstances you need the particle
aspect. You can't throw away either one.
Q: "The wave-particle duality model allows us to make a similar connection
between wavelength and color for photons. Thus, we can also characterize
photons by their wavelengths." Photons have wavelengths?
A: Yes.
Q: I don't think I understand the Doppler Shift very well. Using the
example
in the book, a policeman sends out radar waves that reflect back as
shorter
waves. Does this mean that the car does something to the waves like
our
o-zone changes sunlight when it comes into Earth and prevents most
of it
from escaping back into space?
A: This is a different effect than the ozone. The only way I can
think of
to satifactorily explain this would involve drawing a picture and explain
things as I drew the picture. Please drop by my office sometime,
and I'll
be happy to go over this example with any of you!
Q: Chapter 3: If light can have higher energy with higher frequency,
and
wavelength can be independent of this, how can the equation [(velocity
of
light in vacuum)=(frequency)(wavelength)] still hold? Is this
a relativity
thing, or am I mistaken about the fact that frequency can be independent
of
wavelength (I remember Prof Basri mentioning this-- does it only occur
in
special situations, like when the speed of light varies as well--ie,
when it
is not in a vacuum?)
A: Wavelength is NOT independent! It is always true that energy
is
proportional to frequency. It is always true that wavelength
x frequency =
speed of the wave. Thus it must be true that energy is inversely
proportional
to wavelength.
Q: how unique are emission spectrums for various elements? Are
they all
completely different, and can the spectrum of one element be confused
for,
say, the spectrum of two other elements mixed together? In other
words,
looking at the absorption spectrum of a mixture of several gases, is
it
possible to determine exactly which elements are present in that mixture,
or
is there some ambiguity in the readings?
A: The spectral features of any atom or molecule or ion are COMPLETELY
unique and give astronomers an incredibly accurate fingerprint indicating
the composition of whatever we're looking at.
Q: How does one tell what sort of gas is emitting light?
A: You check to see where the spectral lines are when you break up the
with a prism, the look up which elements or compounds or ions emit
at those
spectral lines.
Q: If light is called "electromagnetic", does it mean that it is magnetic?
i
don't really think of it as attracting or repelling other things.
*A: The amplitude of the EM waves in light is so small that it would
not
affect anything macroscopic. However, when a photon encounters
an electron,
it can scatter off of the electron, and send the electron off in some
direction
like two billiard balls colliding. But it is not because the
photon
physically struck the electron. The electron is a charged particle,
and
thus
when the photon passed by, the electromagnetic field of the photon
and the
electric field of the electron perturbed each other.
Q: I've actually been thinking about this subject for quite awhile.
I've
been wondering for one thing and no one I've asked has known the
answer. How does diffraction work?
*A: I can't really figure out how to answer this without drawing a picture
(more than one color pen would help) and explaining the picture as
I go.
Please drop by my office hours sometime, and I'll be glad to go over
this
topic with you!
Q: What does a prism (a glass pyramid?)
do to separate all the light into a spectrum. While I was reading
I become
very interested in lights property of radiation. I had always
taken for
granted its ability to travel through space. I would like to
further
discuss the disturbances in the electric and magnetic fields that cause
this incredible motion.
*A: These are both pretty involved topics, which can't really be answered
quickly. Again, please drop by my office hours, and I'd love
to chat about
these topics.