Download Properties of Light

The Nature of Light
Electromagnetic Waves and the Electromagnetic Spectrum
What is Light?
Electric charges repel or
attract each other.
+
+ +
This attraction / repulsion
travels at the speed of light.
If you move, say, an electron
the change in force takes
time to travel into the
Universe.
This change in the force field
is an electromagnetic wave.
—
— —
+
—
Light: Waves in What?
A water wave is a disturbance in water’s surface.
A sound wave is a variation in the air pressure.
Light is fundamentally
different.
Light doesn’t need a
substance to travel
through.
Light can travel thru
the vacuum of space!
Wave Properties
The Range of Visible Light
Visible light comes in many colors. Each color has
a particular wavelength and frequency.
Red: longest wavelength
& lowest frequency
Violet: shortest wavelength
& highest frequency
The Range of Visible Light
The human eye is most sensitive to green light,
with a wavelength of about 500 nanometers.
(1 nanometer = 1 nm = 1 billionth of a meter.)
The reddest light has a
wavelength near 700 nm.
The most violet light has a
wavelength near 350 nm.
The eye is sensitive to a very
narrow range of wavelengths!
The unit of frequency on this diagram is the Hertz.
1 Hertz = 1 Hz = 1 wave per second.
The unit of wavelength is the meter, but the range spans
from thousands of meters (km) to trillionths of a meter (pm).
Electromagnetic Waves
The speed of a wave is related to the wavelength
and the frequency:
speed = wavelength × frequency
The speed of EM waves is a constant, so
frequency and wavelength are inversely related:
•  High frequency waves have long wavelengths.
•  Low frequency waves have short wavelengths.
The Speed of Light
The speed of light (often abbreviated as “c”):
c = 300,000 kilometers per second
This is the same speed for all EM waves:
Ø  radio waves have the same speed as visible light
Ø  gamma rays travel at the same speed as x-rays
Ø  ultraviolet & infrared light have the same speed
The Energy of Light
Light carries energy. The amount of energy carried
by a single “packet” of light depends only on the
wave’s frequency.
The energy (E) is related to frequency (f):
E = h× f
Higher frequency means more energy per wave.
“h” is a tiny quantity known as Planck’s constant.
Individual light waves carry a tiny bit of energy.
Which of the following is not a form of light?
A.  radio waves
B.  x-rays
C.  ultraviolet light
D.  All of the above are forms of light.
E.  None of the above are forms of light.
Which has the shortest wavelength?
A.  radio waves
B.  x-rays
C.  ultraviolet light
D.  blue light
Which has the lowest frequency?
A.  radio waves
B.  x-rays
C.  ultraviolet light
D.  blue light
Which carries the highest energy?
A.  radio waves
B.  x-rays
C.  ultraviolet light
D.  blue light
Which travels at the highest speed?
A.  radio waves
B.  x-rays
C.  ultraviolet light
D.  All travel at the same speed.
Temperature Scales
In the U.S. and Trinidad & Tobago, the common
temperature scale is the Fahrenheit scale:
Water freezes at 32o F and boils at 212o F.
The rest of the world uses the Celsius scale:
Water freezes at 0o C and boils at 100o C.
In astronomy, we discuss temperatures far beyond
the range of everyday experience.
Kelvin Temperature Scale
•  No negative Kelvin temperatures
•  No maximum Kelvin temperature
•  Room temperature is about 300 Kelvin
Cores of stars
(hotter than 10 million Kelvin)
yikes!
Surface temperatures of stars
(3000 – 50,000 Kelvin)
Dark interstellar gas cloud
(10 - 100 Kelvin)
Overall temperature of Universe
2.73 Kelvin
Absolute zero = zero Kelvin
(coldest possible temperature)
brrrr!
Thermal (or Blackbody) Spectrum
A hot, dense object will emit a continuous range of
colors known as a thermal spectrum (just like the
hot filament of the light bulb).
This type of spectrum is also called a blackbody.
Intensity
short
Wavelength
long
Blackbody Temperature
The height and wavelength of the peak shift as the
temperature increases: brighter and bluer…
Blackbody Temperature
The frequency or wavelength of the peak of the
blackbody curve depends on the temperature of
the emitting object.
The higher the temperature, the higher the energy
and frequency of the peak. This means that the
wavelength of the peak gets shorter.
By observing an object’s thermal spectrum, we
can determine the temperature of the object.
A 15,000 K star is brightest in which part of the
electromagnetic spectrum?
A.  radio
B.  infrared
C.  visible light
D.  ultraviolet
What is the visual color of a star like the Sun?
A.  Greenish
B.  Bluish
C.  White-ish
D.  Reddish
Which object is hotter?
A.  Object A
B. Object B
Which of these objects is largest in size?
(Think back to luminosity vs. temperature vs. size)
A.  Object A
B.  Object B
C.  Object C
D.  Object D
If an astronomer wanted to find the temperature
of a distant object, which feature of the objectʼs
spectrum should be examined?#
A.  which spectral lines are present#
B.  the total intensity of the objectʼs spectrum#
C.  the wavelength of the peak of the spectrum#
Atomic Structure
You will often see an atom drawn with electrons
orbiting the nucleus like planets:
However, electrons do not move on fixed orbits…
Instead the electrons occupy an electron cloud.
The electron orbits do have unique energy values.
A Simple Spectrum
Imagine a toy hydrogen atom with 1 proton and
1 electron. The electron is only allowed to have
two energy values… let’s say 1 unit and 3 units.
upper energy level = 3 units
electron
proton
lower energy level = 1 unit
A jump by an electron from one energy level to the
other creates or destroys a photon, a single
“packet” of light energy.
Photons can have different amounts of energy,
depending on the size of the electron’s jump:
2 energy units
added to atom
by photon (light)
photon absorbed
2 energy units
carried away by
photon (light)
photon emitted
A Simple Spectrum
In the case of our toy 1-electron atom, how much
energy does the electron need to gain to jump up?
E = 3 units
E = 1 unit
electron
proton
A Simple Spectrum
How much energy does the electron lose when it
jumps down?
E = 3 units
E = 1 unit
proton
electron
If we observe a cloud of these “toy” 2-level atoms,
we would see it emit light of just a single energy.
(only one wavelength, frequency, and color, too.)
The emission spectrum of this toy 2-level atom is a
single bright line, called an emission line.
The line’s energy/wavelength/frequency/color
relates to the difference in energy between the
two electron energy levels.
Energy Levels of Hydrogen
Real electrons in real hydrogen atoms have a very
large number of energy levels.
13.6 eV
Jumps to energy level 1 are
in the ultraviolet.
Jumps to energy level 2 are
visible to the human eye.
Jumps to energy level 3 = ?
?
visible
ultraviolet
The electron-volt (eV) is the commonly used
unit of energy for electrons and atoms.
12.8 eV
12.1 eV
10.2 eV
0 eV
Energy Levels of Hydrogen
What process occurs if the electron jumps up?
è Absorption
13.6 eV
What happens if the electron
gains enough energy and
jumps past the highest
energy level?
12.8 eV
12.1 eV
10.2 eV
è Ionization
0 eV
An ionized atom (an ion) has lost an electron.
Free electrons may later recombine with an ion.
The electrons in an atom can be excited in a
variety of ways:
•  absorption of light energy (radiative energy)
•  collision with fast-moving atoms (kinetic energy)
•  collision with free-floating electrons (kinetic)
Excited electrons drop to lower energy levels and
release the energy as light.
A spectrum with bright lines on a dark background
is known as an emission spectrum.
Absorption of light energy is the exact opposite of
emission of light energy.
Emission lines and absorption lines match exactly.
The light absorbed by the atoms is missing from
the spectrum. An absorption spectrum appears
as dark lines against a continuous background.
hydrogen
Clues in the Light
The spectrum of a chemical element is unique
and acts like a fingerprint.
The spectral lines in the light from an object tell
us the composition of the object!
Here are the spectra of 3 different gases:
Thermal spectrum:
hot, dense matter
Emission spectrum:
hot, low-density gas
Absorption spectrum:
a thermal spectrum
passing thru cooler
cloud of gas
Which type of spectrum would be emitted by a
glowing gas cloud heated by a nearby O star?#
O star#
A.  A continuous spectrum#
B.  An absorption spectrum#
C.  An emission spectrum#
The spectrum of typical emission nebula#
A normal star has a very hot core surrounded by a
cooler atmosphere.#
#
Normal stars emit what kind of spectrum?#
A.  Emission#
B.  Absorption#
C.  Continuous#
The spectrum of Procyon (A-type dwarf star)#
The spectrum of the Sun#
The spectrum of Arcturus (K-type giant star)#
Doppler Shift: Observer and Source
So far the light source and observer have been
considered stationary relative to one another.
If the source is approaching the observer (or vice
versa), light waves will arrive more often. The
light waves appear to be compressed.
What happens to the apparent color of the light?
If the source is receding from the observer (or vice
versa), light waves will arrive less often. The
light waves appear to be stretched out.
What happens to the apparent color of the light?
Doppler: a Subtle Change in Color
The motion of an object changes the color of light
emitted by the object.
Approaching, light waves
are compressed and
blue-shifted.
Receding, the light waves are
stretched and red-shifted.
This effect only depends on motion, not distance.
It is visible for nearby planets or distant galaxies.
You observe the spectrum of a distant unknown
object. You recognize a spectral line of helium at a
wavelength of 700 nm. In the lab (at rest), this line
has a wavelength of 512 nm.#
#
What can you say about the objectʼs motion?#
A.  It must be moving toward us.#
B.  It must be moving away from us.#
C.  It must be moving sideways relative to our view.#
D.  It cannot be moving at all relative to us.#