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Chapter 5: Light. Light!. Measuring the speed of light. Early attempts to measure the speed of light were done in 1638 by an apprentice of Galileo Hilltop to hilltop around Padua Italy using hand lanterns “The speed of light is at least 10 times faster than the speed of sound.”. - PowerPoint PPT Presentation
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Light!Measuring the speed of lightEarly attempts to measure the speed of
light were done in 1638 by an apprentice of GalileoHilltop to hilltop around Padua Italy using hand lanterns
“The speed of light is at least 10 timesfaster than the speed of sound.”
Rømer estimates the speed of light1676…Olaus RømerEclipses of Io by Jupiter. Found speed to be finite but very large (didn’t know diameter of Earth’s orbit)
By the 1800’s two Frenchmen were able to measure the speed
of light with some accuracy1850…Fizeau & Foucault measure speed to be
about300,000 km/s
The Speed of Light in vacuum is a fundamental constant of
the universeThe speed of light in vacuum is the same for all observers everywhere in the universe regardless of their circumstances. We define the speed of light to be
c = 299792458 m/s exactly.For most calculations, though, we can use 3.00 x108 m/s
We can use the speed of light to define distances
seconds2997924581intravelslightdistancemeter1
The meter is defined as
The lightyear is defined as yearoneintravelslightdistance1ly
The year used in the definition is the sidereal year: 365 days 6 hours 9 minutes and 9.5 seconds
When light travels through anything other than vacuum
it moves slowerWe define the index of refraction of a material to be the ratio of the speed of light in vacuum to the speed of light in the material
vcn
Just as a cork bobbing in
water creates waves in the
water, charges “bobbing” in space create electric and magnetic
waves
Basic Properties of WavesWavelength = l in metersFrequency = f in cycles per second or Hertz (Hz)Speed = v in meters per second
lfc
Each “color” is characterized by its
wavelength
Using c = lf we can see that the frequency of visible light is in the 1014 Hz range
Visible light is only a very small part of the
Electromagnetic Spectrum
Watch ClassAction Light & Spectra module EM Spectrum Module animation
Photon Torpedoes Away!
l
hhcE sJh 3410625.6
Light also behaves like a particle: a photon
c = speed of light = frequencyl = wavelength
But didn’t we say that light was a wave? What’s going on? How can it be both?
To understand how light can be both a wave and a particle, we
need new physicsQuantum Mechanics deals with the very small like atoms, molecules, subatomic particles and light particles.
The Bohr AtomAn early model of the atom. Like a mini-solar system, the electrons orbit around tiny nucleus The Nucleus: Protons, Neutrons and 99.98% of the mass
The problem is, it doesn’t work! According to electromagnetic theory, the electron will radiate away all its energy and collapse into the nucleus in a fraction of a second.
The Quantum Mechanical Atom
The Heisenberg Uncertainty Principle of quantum mechanics says you can’t tell where the electron is, only the probability of where it might be. The electron is “delocalized”.
Light comes from Electron Transitions
The energy of the emitted photon is exactly equal to the difference in energy between the two states
photon12 EhcEE l
Absorption and emission lines have the same
wavelengths The same states are involved in emission and absorption. The only difference between emission and absorption is the way the electron is changing. For emission the electron loses energy and thus goes from a high energy state to a low energy state. For absorption, the electron gains energy and so goes from a low energy state to a high energy state.Three Views Spectrum Demonstrator in ClassAction Light & Spectra module
The Solar Spectrum has lots of absorption lines
All the naturally occurring elements are present in the Sun
The Doppler Effect
01 1 ll
cvs
+ If the source is moving away from the observer- If the source is moving towards the observer
The light is redshifted if the source is moving away from the observer, blueshifted if it is moving towards
What do we mean when we say something is hot?
On a microscopic scale, temperature is a measure of how fast things are moving
In astronomy, we use the absolute
temperature scale
Absolute zero is the temperature at which all motion stops. Quantum mechanics says that isn’t possible so you can never reach absolute zero
The Blackbody Spectrum
The hotter the object is, the shorter the wavelength of maximum intensity. The hotter the object is, the greater the total intensity of the light it emits is.