Transcript pptx - Penn State University

Light
Light as a Wave
Light (or electromagnetic radiation), can be thought of as either a
particle (a photon) or a wave. As a wave, light has
• a wavelength,  (distance between waves)
• a frequency,  (number of waves passing you each second)
• a speed, c =   (300,000 km/s; all wavelengths have same speed)
• an energy, E = h  (where h is just a constant)
Note that because the speed of light is a constant, , , and E are
linked: if you know one, you can compute the other two.
The Electromagnetic Spectrum
(optical)
Atmospheric Windows
Visible, radio, and some infrared and some UV wavelengths
penetrate the atmosphere and reach the ground from space.
Other wavelengths (X-ray, gamma ray, some IR, some UV) are
blocked by the atmosphere.
The Doppler Shift
The wavelength emitted by an object is not always the wavelength
you observe. If an object is moving towards you, the wavelength you
see is shorter (bluer) than the wavelength of the light emitted by the
object. This is called a blue shift. Conversely, if an object is moving
away from you, you will see light that is redshifted compared to the
light that was emitted.
If the velocity is higher,
then the Doppler shift is
larger. So we can
measure the velocity of
a star, galaxy, etc. from
its Doppler shift.
Inverse Square Law of Light
When light is emitted by an object (a light bulb, star, etc.), it moves
away in all directions, and spreads out over a larger and larger area.
As a result, an object appears fainter when viewed from a larger
distance. The inverse square law of light describes how much
fainter an object appears at larger distances:
brightness  1/d2 where d=distance
So if a light source moves 2
times farther away, its
brightness changes by ¼, i.e., it
becomes 4 times fainter.
Inverse Square Law of Light
Scattering of Light
Air molecules in the Earth’s atmosphere scatters light. Shorter
wavelengths (bluer) are scattered more than longer wavelengths
(redder). When we look at the sky, we see the bluer photons
scattering in the atmosphere. That’s why the sky is blue.
Scattering of Light
Near the horizon, light travels a longer path through the
atmosphere before reaching us. As a result, the light experiences
so much scattering that the most of the blue light is lost. This is
why the sky appears red near the horizon at sunrise and sunset.
1st Type of Radiation: Blackbody (or Thermal)
Every solid object, liquid, and dense gas in the universe produces
backbody radiation (also known as thermal radiation). This light is
produced at all wavelengths, which is called a continuous spectrum.
At visible wavelengths, this spectrum appears as the rainbow.
A light bulb is an example of a source of blackbody radiation. When
its light passes through a prism, we see the continuous spectrum
(full rainbow).
1st Type of Radiation: Blackbody (or Thermal)
The blackbody spectrum is very sensitive to temperature.
At higher temperatures:
• the peak of the spectrum
appears at shorter (bluer)
wavelengths
• more light is produced
(the object is brighter),
where the total amount of
light is proportional to T4
http://astro.unl.edu/naap/blackbody/animations/blackbody.html
Part of the heat we feel from a fire is blackbody radiation
emitted by the fire and absorbed by our bodies. (The rest
of the heat that we feel is produced by the fire raising the
temperature the surrounding air, which in turn raises our
temperatures.)
In an atom, electrons live in levels that have specific energies.
E4
E3

E2
E1
E1

E2
E3
E4
The level with the lowest energy is called the ground state.
E4
E3

E2
E1
E1

E2
E3
E4
-
A collision with another atom can cause the electron to
jump to a higher level, called an excited state.
-
E4
E3

E2
E1
E1

E2
E3
E4
-
An electron in an excited state can spontaneously drop to one
of the lower levels, releasing a photon (a particle of light).
-
E4
E3

E2
E1
E1

E2
E3
E4
-
When an electron moves from one level to another, this is
called a transition.
-
E4
E3

E2
E1
E1

E2
E3
E4
-
The photon’s energy equals the difference between the
energies of the old and new levels.
-
Ephoton = E3-E2
E4
E3

E2
E1
E1

E2
E3
E4
Ephoton = E3-E2
The transitions A, B, and C produce 3 emission lines at different
wavelengths. Which transition produces the bluest emission line?
A
B
C

E1
E2
E3
E4
Transitions between different pairs of levels produce
photons at different energies (or wavelengths).
4-1
3-1
2-1

E1
E2
E3
2-1
3-1
E4
4-1
This is an
emission line
spectrum
Different elements and molecules have different energy
levels, so they produce emission lines at different
wavelengths.


E1
E2
E3
E1
E4
E2 E 3 E
4
Emission Line Spectra
Since every element has a different set of energy levels in which
electrons live, the emission line spectrum of every element is different.
They are as unique as fingerprints.
The orbital with the lowest energy is called the ground state.
E4
E3

E2
E1
E1

E2
E3
E4
-
A collision with another atom can cause the electron to
jump to a higher level, called an excited state.
-
E4
E3

E2
E1
E1

E2
E3
E4
-
To produce emission lines, a gas needs to have electrons that
are excited to the higher levels. One way to excite electrons is
heating the gas since this increases collisions between atoms.
There’s a second way of exciting an electron to a higher level.
Begin with a source of light, a blackbody producing a
continuous spectrum in this example.
Shine the light on a cloud of gas (or a single atom).

E1
E2
E3
If one of the approaching photons has the exact energy
needed to excite the electron to a higher level (E2-E1, E3-E1,
etc.), then the electron can absorb that photon and jump to a
higher level.

E1
E2
E3
Most of the light passes through the gas, but the photons that
were absorbed by electrons are now missing. These missing
photons in the spectrum are called absorption lines.

E2-E1
E1
E2
E3
An absorption line spectrum is produced when light passes through a
cloud of gas.
Absorption Line Spectrum from the Sun
Why does the Sun produce absorption lines?
(1 Million K)
Continuous, absorption, and emission line spectra
continuous
blackbody
absorption line
blackbody
web site illustration
emission line
The Doppler Shift
Earlier, we found that light from an object appears blueshifted if it is
moving towards the observer, and its light appears redshifted if it is
moving away. This Doppler shift applies to any absorption or emission
lines in the spectrum of the moving object.
web site illustration