Basic properties of light and matter.

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Chapter 5

• Basic properties of light and matter.
• What can we learn by observing light from distant
  objects?
• How do we collect light from distant objects?

Solar Spectrum
2
Light and Matter

• Why do we need to study light?
• Light is the messenger that carries information
  of distant astronomical objects to us.
• We will first talk about what is light and
  matter, and how they interact. Then we can
  understand
• How can we learn about the composition of cosmic
  objects by observing the light they emit.
• How can we study the motion of cosmic objects by
  observing the light they emit.
• How can we tell about the environment
  (temperature, density, etc.) of the cosmic
  objects by observing the light they emit.

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What is Light?

• Light is a form of energy (radiative energy).
• White light is actually the mixture of equal
  intensity of light with different color (in the
  visible wavelength regime).
• Different color of light carry different amount
  of energy.
• Different wavelength.
• Different frequency.
• Light has dual personality
• Light as waveelectromagnetic waves.
• Light as particlephotons.

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How do we generate light?

• Light can be generated by accelerating (NOT just
  moving with a constant velocity) an electric
  charge
• For example, if we make an electric charge move
  in a circle with a constant orbital speed, then
  this accelerating electric charge will emit
  light.
• To emit red light, this electric charge needs to
  complete 500 trillions orbits in one second (5
  1014 circles/second, or a frequency of 5 1014
  Hertz)
• Moving a electrical charge back and forth alone a
  straight line also workantenna
• Light can also be generated by heating up an
  objectthermal radiation.

Conservation of Energy is at work hereIt takes
energy to keep accelerating the electric charges.
These energy (for example, chemical potential
energy stored in the battery) are converted into
the radiation energy of light!
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Electric and Gravitational Fields

• Gravitational Field
• Matters change the property of the space around
  it. The property of space associate with the
  presence of matter is call the gravitational
  field.
• Electric Field
• Electric charges change the property of the
  space around it. The property of space associate
  with the presence of electric charges is call the
  electric field.
• The gravitational (electric) field interacts with
  a second matter (electric charge) by exerting a
  force on the second matter (electric charge).

r
m1
m2
e1
e2

• The formula for the electric force between two
  charges is identical to the Newtons law of
  gravity in form. If we replace mass m1 and mass
  m2 with charge e1 and charge e2, and appropriate
  constant C, then we get the formula for electric
  force.

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Electric and Gravitational Fields

• One BIG difference between gravitational and
  electric force is that there is no negative mass.
  Gravitational force is always attractive.
• Gravitational force is the dominant force acting
  between astronomical objects.
• The net electric charges on most of the objects
  are all very close to zero. Therefore, we do not
  experience electric force very much most of the
  time.
• Electric force is important inside the atoms.

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Lights Dual Personality

• We can think of light as wave
• When light travels through space, the electric
  field along the path of the light would vary in
  the form of a wave.
• If we place a row of electrons along the path,
  then the electrons would move up and down as a
  line of corks in wavy water.
• The wavelength of the light wave determines the
  energy and color of the light.
• The speed of light is 300,000 km/sec in vacuum,
  It does not depend on its wavelength.
• The diffraction and interference phenomena of
  light is a characteristic property of wave.

• We can also think of light as particle
• Some properties of light resemble that of
  particles. For example, we can count the
  number of photons we receive.
• Photons can have different energy.
• Photons DO NOT have mass!
• The energy each photon carries is related to the
  color (wavelength, frequency) of the photon.

Click on image to start animation
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The Electromagnetic Spectrum
Because of the close association between light
and electrical charges and magnetism, light waves
are also called electromagnetic wave.
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• The speed of light is 300,000 km in vacuum.
• It is not infinitely fast
• Nothing can be accelerated to speed higher than
  the speed of light.
• Speed of light does not depends on color. It is
  the same for all the colors.
• The energy light carries depends on its color.
  Red is low energy. Blue is high energy.

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Matter

• Ancient Greeks considered that matters are
  composed by tiny, indivisible particles called
  atoms.
• In modern physics, we have identified more than
  100 different chemical elements
• Each element is made of a different type of atom,
  
• Example of elements are hydrogen, helium,
  carbon, oxygen, iron, gold, silver, etc.
• Atoms have a nucleus, made up of protons and
  neutrons, with positive charges, and a electron
  clouds surrounding the neucleus.
• Protons each proton carries one positive
  electrical charge.
• Neutrons neutrons are electrically neutral. They
  dont carry any electrical charge.
• Electrons each electron carries one negative
  electrical charge.
• Proton and Neutron have about the same mass.
• Electrons are much lighter than protons and
  neutrons.

Hydrogen atom has only one proton and one electron
Helium atom has two protons and two neutrons in
its nucleus, surrounded by two electrons
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Chemical Properties of Elements

• The chemical property of an element is determined
  by the total electrical charge (or the number of
  protons) of the nucleus.
• Elements with the same number of protons but
  different number of neutrons are called isotope.

12
Interaction Between Light and Matter

• Matter can emit, absorb, transmit, or
  scatter/reflect light.
• Emission
• Black Body Emission An object with a finite
  temperature will emit light with a spectrum
  described by a black body spectrum.
• Spectral Emission The atoms of the object can
  absorb only light at certain frequency, and then
  re-emit light in these frequencies in all
  direction.
• Absorption Matter can absorb light, result in
  the increase of its temperature (conversion of
  radiative energy into thermal energy).
• Transmission Some matters (like glass) allow
  light to propagate through. The speed of light in
  these matters will be different from that in the
  vacuum. The direction of propagation will be
  changed also.
• Reflection/scattering Photons may bounce off the
  surface of some matters, like mirror (a thin
  coating of aluminum on the surface of glass).
• Reflection When the incident light travels
  toward the matter in the same direction are
  bounced toward a same general direction, or
• Scattering When the light that was bounced off
  the surface of an object is sent into random
  direction.

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Examples of Light/Matter Interaction

• Something that is white (to human eyes) means it
  reflect all the visible light
• Something that is black (to human eyes) means
  that it absorbs all the visible light.