Download Introduction to the physics of light

PHYS
206
Matter and Light
• At least 95% of the celestial information we
receive is in the form of light.
• Therefore we need to know what light is and
where it comes from.
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Electric and Magnetic Fields
• Electrical charges and magnets alter the region of space around them so that they can exert
forces on distant objects.
• James Clerk Maxwell proposed that if a changing magnetic field can make an electric field,
then a changing electric field should make a magnetic field.
• A consequence of this is that changing electric and magnetic fields should trigger each other
and these changing fields should move at a speed equal to the speed of light.
• Maxwell also said that light is an electromagnetic wave.
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Electromagnetic Radiation
• Light, electricity, and magnetism are manifestations of the same thing called
electromagnetic radiation.
• This energy also comes in many forms that are not detectable with our eyes such as
infrared (IR), radio, X-rays, ultraviolet (UV), and gamma rays.
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Properties of EM Radiation
• It can travel through empty space. Other types of waves need some sort of
medium to move through: water waves need liquid water and sound waves need
some gas, liquid, or solid material to be heard.
• The speed of light is constant in space. All forms of light have the same speed
of 300,000 kilometers/second in space (often abbreviated as c).
• The forms of light are Gamma rays, X-rays, Ultraviolet, Visible, Infrared,
Radio.
• A wavelength of light is defined similarly to that of water waves---distance
between crests or between troughs. Visible light (what your eye detects) has
wavelengths 4000-8000 Ångstroms. 1 Ångstrom = 10-10 meter. Visible light is
sometimes also measured in nanometers: 1 nanometer = 10-9 meter. Radio
wavelengths are often measured in centimeters. The abbreviation used for
wavelength is the greek letter lambda .
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Intensity and Energy
• The energy of the EM
radiation depends only on the
wavelength (frequency), the
shorter the wavelength, the
higher the energy (blue is
more energetic than red).
• The type of EM radiation produced by an object will also depend on
its energy (temperature). Temperature is a measure of the random
motion (or energy) of a group of particles. Higher temperature (T)
means more random motion (or energy).
• Blue stars have a higher temperature than red stars.
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The direction of light
propagation can be changed at
the boundary of two media
having different densities.
This property is called
refraction.
Different frequencies will
break differently at the
interface.
To decode the information
stored in light, you pass the
light through a prism or
diffraction grating to
create a spectrum. If white
light is examined, then the
spectrum will be a rainbow.
Refraction
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Spectrum of visible light
• Continuous spectrum
• Discrete spectrum
• Emission spectrum
• Absorption spectrum
• Hot objects give thermal spectrum
(continuous spectrum).
• White light has a continuous spectrum.
• Even though all the colors are present
in the spectrum we can still see a
different color.
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Temperature Dependence of
Spectra
• All “hot” objects give out light.
• The shape of the spectrum depends
only on the temperature of the object.
• As the temperature of an object
increases, more light is produced at all
wavelengths than when it was cooler.
• As the temperature of an object
increases, the peak of thermal spectrum
curve shifts to smaller wavelengths.
• When you add up all of the energy of all
of the square meters on the object's
surface, you get the luminosity---the
total amount of energy emitted every
second by the object.
• Luminosity is proportional to the fourth
power of temperature.
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Discrete Spectrum
Emission spectra are produced by thin gases in which the atoms do not
experience many collisions (because of the low density).
A continuum spectrum results when the gas pressures are higher, so that lines
are broadened by collisions between the atoms until they are smeared into a
continuum.
An absorption spectrum
occurs when light passes
through a cold, dilute gas and
atoms in the gas absorb at
characteristic frequencies.
Where do these
characteristic frequencies
come from?
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Atoms and Matter
• Niels Bohr provided a solution (Bohr atom).
• Electrons can be only found in energy orbits of a certain size (quantization).
• As long as the electron is in one of those special orbits, it would radiate no
energy.
• The massive but small positively-charged
protons and massive but small neutral neutrons
are found in the nucleus.
• The small, light negatively-charged electrons
move around the nucleus in certain specific
orbits (energies).
• In a neutral atom the number of electrons =
the number of protons.
•All atoms with the same number of protons in
the nucleus are called an element.
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Bohr Atom
Elements are divided into subgroups called isotopes based on
the number of protons AND
neutrons in the nucleus.
All atoms of an element with the
same number of neutrons in the
nucleus are of the same type of
isotope.
When an atom has an extra
positive or negative charge, than
it is called an ion.
Electrons have only certain energies corresponding to particular distances from
nucleus. As long as the electron is in one of those energy orbits, it will not lose or
absorb any energy.
The orbits closer to the nucleus have lower energy.
Atoms want to be in the lowest possible energy state called the ground state.
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Emission Spectrum
An emission line is produced by an atom in an “excited” energy state.
The electron is not in as low an energy orbit as possible.
In order to go to a lower energy orbit, the electron must lose energy of a certain
specific amount.
The energy of photon =
the difference in energy
of the energy orbits.
The intensity depends
on the density and
temperature of the gas.
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Absorption Spectrum
An absorption line is produced
when a photon of just the right
energy is absorbed by an atom,
kicking an electron to a higher
energy orbit.
Other photons moving through
the gas with the wrong energy
will pass right on by the atoms in
the thin gas. They make up the
rest of the continuous spectrum
you see.
The more atoms undergoing a
particular absorption transition,
the darker the absorption line.
The strength of the absorption
line depends on the density and
temperature.
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Atmospheric Windows
Our atmosphere also absorps some of the radiation as well.
Absorption is maximum for X-rays and UV rays, and minimum for visible and radio
radiation.