Download Properties of Light

The Nature of Light
Electromagnetic Waves and the Electromagnetic Spectrum
What is Light?
Electric charges repel or
attract each other.
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This attraction / repulsion
travels at the speed of light.
If you move a proton here,
the change in “force” will
take time to travel out into
the Universe.
This change in the force field
is an electromagnetic wave.
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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!
Electromagnetic Waves
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:
• If frequency increases, wavelength decreases.
• If wavelength increases, frequency decreases.
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).
The Speed of Light
The modern measured value for the speed of light:
c = 300,000 kilometers per second
This is the same speed for all EM waves:
¾ radio waves have the same speed as visible light
¾ gamma rays travel at the same speed as x-rays
¾ 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 of the above.
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 the peak of the spectrum shift as
temperature changes.
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 true visual color of a star like the Sun?
A. Violet
B. Bluish
C. White
D. Reddish
Which object is hotter?
A. Object A
B. Object B
Which of these objects is largest?
(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 object’s
spectrum should be examined?
A. which spectral lines are present
B. the total intensity of the object’s spectrum
C. the wavelength of the peak of the spectrum
Atomic Structure
You will often see an atom drawn this way.
However, electrons do not move on fixed orbits
like planets. Instead, they can be found buzzing
around anywhere near the nucleus.
The electrons are said to occupy an electron cloud.
A Simple Spectrum
Imagine a toy hydrogen atom with 1 proton and
1 electron. The electron is only allowed to have
two amounts of energy…
electron
proton
lower energy level
upper energy level
A jump by an electron from one energy level to the
other creates or destroys a photon, a single
“packet” of light.
Photons can have different amounts of energy,
depending on the size of the electron’s jump:
photon absorbed
photon emitted
In this toy 2-level atom, the electron can only gain
or lose a specific amount of energy. This means
that the atom can only emit light of one energy!
(Only 1 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 color corresponds to the electron’s jump.
The color of the spectral line is determined by the
difference in energy between two energy levels.
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, too)
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.
The spectrum of an element is unique and acts
like a fingerprint. The spectral lines in the
light from an object tell us the composition!
The same is true of molecules… here’s H2.
The energy lost by an emitting electron equals the
energy absorbed by that electron jumping up.
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
You observe the spectrum of a distant unknown
object. You recognize emission lines of carbon and
absorption lines of iron.
What can you say about the object’s composition?
A. It must contain carbon, but no iron.
B. It must contain iron, but no carbon.
C. It contains both carbon and iron.
D. It cannot contain either carbon or iron.
Which type of spectrum would be emitted by a
glowing gas cloud heated by an O-type 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 spectra?
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 we have talked about the light source and
observer being stationary relative to each other.
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 1024 nm. In the lab, this line has a
rest wavelength of 512 nm.
What can you say about the object’s motion?
A. It must be moving toward us fast!
B. It must be moving toward us slowly.
C. It must be moving away from us fast!
D. It must be moving away from us slowly.
E. It cannot be moving toward or away from us.