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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
Volume
2 meters
1 m
eter
Side view of sphere
Increase r by a factor of 2. What happens to volume?
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?
Outline
• Short review on interpreting equations
• Light– Suggested reading: Chapter 5.1-5.2 and 5.6-
5.8 of textbook
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.
Particle
• When thinking about the speed of light, we think of a photon as a wave.
Lecture Experiment
X Y
X Launches a photon every 5 seconds.How often does Y get hit by a photon?
photon
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
http://en.wikipedia.org/wiki/File:Roemer.jpg
• 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?
Determining the Speed of Light
• Galileo tried unsuccessfully to determine the speed of light using an assistant with a lantern on a distant hilltop
• 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
• What would happen if the stationary mirror was actually moving towards the rotating mirror?
Waves
• A review of waves
Light is electromagnetic radiationand is characterized by its wavelength ()
Frequency and Wavelength
f or unit time
1unit length
The Greek letter “nu” and not the letter “v”
• 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
Finish line
v = 100 km/hr
Replace cars with lines
• 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
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.
Demo with snapping rope
Interference - destructive
t=1
t=2
Interference - destructive
t=3
t=4
Interference
?
t=1
t=2
Interference
?
?t=3
t=4
Video
http://phys23p.sl.psu.edu/CWIS/SPT--BrowseResources.php?ParentId=189
Interference
Waves in two dimensions
x
x
x x
x x
x x
x xx
x
The electromagnetic spectrum
• 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
• Which of the following has the highest frequency?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays
• Which of the following has the highest wavelength?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays
• Which of the following has the highest speed?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays
• 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
• How is the energy of a photon related to its frequency?
The dual nature of light
• Particle
• Wave
Particle
What would you expect if instead of a laser beam you used yellow spray paint beam?
Draw it!
Wave
x
x
x x
x x
x x
x xx
x
x
x
x x
x x
x x
x xx
xThe amplitude of the wave in the middle is the highest
“Projection screen”
The atom and light
• 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
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.
