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Basic properties of light and matter.


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? – PowerPoint PPT presentation

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Title: Basic properties of light and matter.

Chapter 5
  • Basic properties of light and matter.
  • What can we learn by observing light from distant
  • How do we collect light from distant objects?

Solar Spectrum
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
  • 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.

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.

How do we generate light?
  • Light can be generated by accelerating (NOT just
    moving with a constant velocity) an electric
  • For example, if we make an electric charge move
    in a circle with a constant orbital speed, then
    this accelerating electric charge will emit
  • 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
  • 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!
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
  • 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).

  • 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

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
  • Electric force is important inside the atoms.

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
The Electromagnetic Spectrum
Because of the close association between light
and electrical charges and magnetism, light waves
are also called electromagnetic wave.
  • 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.

  • Ancient Greeks considered that matters are
    composed by tiny, indivisible particles called
  • 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

Hydrogen atom has only one proton and one electron
Helium atom has two protons and two neutrons in
its nucleus, surrounded by two electrons
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.

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
  • 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

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.
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