lecture 11 astr 111 – section 002. outline short review on interpreting equations light...
TRANSCRIPT
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Lecture 11
ASTR 111 – Section 002
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Outline
• Short review on interpreting equations
• Light– Suggested reading: Chapter 5.1-5.2 and 5.6-
5.8 of textbook
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Volume
2 meters
1 m
eter
Side view of sphere
Increase r by a factor of 2. What happens to volume?
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Related questions
• What happens to answer if measurements were 1 yard and 2 yards instead of 1 meter and 2 meters?
• What happens if diameter increases by a factor of two?
• How much more paint is required to cover the larger sphere?
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Outline
• Short review on interpreting equations
• Light– Suggested reading: Chapter 5.1-5.2 and 5.6-
5.8 of textbook
![Page 6: Lecture 11 ASTR 111 – Section 002. Outline Short review on interpreting equations Light –Suggested reading: Chapter 5.1-5.2 and 5.6- 5.8 of textbook](https://reader038.vdocument.in/reader038/viewer/2022110322/56649d045503460f949d77cf/html5/thumbnails/6.jpg)
Particle versus Wave
• Light is composed of photons.
• To fully understand light, we need to think of it in terms of both a particle and a wave.
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Particle
• When thinking about the speed of light, we think of a photon as a wave.
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Lecture Experiment
X Y
X Launches a photon every 5 seconds.How often does Y get hit by a photon?
photon
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Light travels through empty space at a speed of 300,000 km/s
• In 1676, Danish astronomer Olaus Rømer discovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth
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http://en.wikipedia.org/wiki/File:Roemer.jpg
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• Light travels at 300,000 km/sec
• About how fast does your car travel in km/hour when you are on the freeway?
• About how fast does your car travel in km/second when you are on the freeway?
• If it takes light 8 minutes to travel from the sun to Earth, how long would it take you to drive?
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Determining the Speed of Light
• Galileo tried unsuccessfully to determine the speed of light using an assistant with a lantern on a distant hilltop
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• In 1850 Fizeau and Foucalt also experimented with light by bouncing it off a rotating mirror and measuring time
• The light returned to its source at a slightly different position because the mirror has moved during the time light was traveling
• d=rate x time again gave c
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• What would happen if the stationary mirror was actually moving towards the rotating mirror?
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Waves
• A review of waves
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Light is electromagnetic radiationand is characterized by its wavelength ()
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Frequency and Wavelength
f or unit time
1unit length
The Greek letter “nu” and not the letter “v”
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• Cars are traveling at 100 km/hr to the right• What would you need to know in order to
be able to tell how often (frequently) a car will pass the finish line?
Finish line
velocity = 100 km/hr
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Finish line
v = 100 km/hr
Replace cars with lines
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• Cars are traveling at 100 km/hr to the right• What would you need to know in order to
be able to tell how often a peak will pass the finish line?
Finish line
v = 100 km/hr
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Finish line
cDistance between peaks
How often peakpasses finish line
How fast wave moves to right
Frequency and wavelength are intimately related for a wave.
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Demo with snapping rope
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Interference - destructive
t=1
t=2
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Interference - destructive
t=3
t=4
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Interference
?
t=1
t=2
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Interference
?
?t=3
t=4
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Video
http://phys23p.sl.psu.edu/CWIS/SPT--BrowseResources.php?ParentId=189
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Interference
Waves in two dimensions
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x
x
x x
x x
x x
x xx
x
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The electromagnetic spectrum
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• Because of its electric and magnetic properties, light is also called electromagnetic radiation
• Visible light falls in the 400 to 700 nm range
• Stars, galaxies and other objects emit light in all wavelengths
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• Which of the following has the highest frequency?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays
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• Which of the following has the highest wavelength?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays
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• Which of the following has the highest speed?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays
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• Which of the following has the highest energy E (h is a constant)?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays
hc
E
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• How is the energy of a photon related to its frequency?
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The dual nature of light
• Particle
• Wave
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Particle
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What would you expect if instead of a laser beam you used yellow spray paint beam?
Draw it!
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Wave
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x
x
x x
x x
x x
x xx
x
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x
x
x x
x x
x x
x xx
xThe amplitude of the wave in the middle is the highest
“Projection screen”
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The atom and light
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• An atom has a small dense nucleus composed of protons (and neutrons)
• Rutherford’s experiments with alpha particles shot at gold foil helped determine the structure
Probing the atom
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Spectral lines are produced when an electron jumps from one energy level to another within an atom
• The nucleus of an atom is surrounded by electrons that occupy only certain orbits or energy levels
• When an electron jumps from one energy level to another, it emits or absorbs a photon of appropriate energy (and hence of a specific wavelength).
• The spectral lines of a particular element correspond to the various electron transitions between energy levels in atoms of that element.
• Bohr’s model of the atom correctly predicts the wavelengths of hydrogen’s spectral lines.