Download Homework Speed of light is now fixed Edwin Hubble

Homework
Due Friday Feb. 03
Seeing the Light Chapter 1
P2, P11, P14, P17, P20, PH2, PM2, PM8
Edward Weston
Speed of light is now fixed
c = 299 792 458 m / s - defines the meter
How fast is fast?
• time to go across lecture hall ~ 10-7 s
• LA to NY ~ 0.01 s
• Earth to Moon ~ 1.3 s
• Sun to Earth ~ 500 s
• nearest star ~ 4.2 years (Lightyears)
• furthest object seen ~ 13.2 x10 9 Ly
Edwin Hubble
• 1929 - Measured spectra (color)
of stars, and found the
further away they were, the
more the spectra was
redshifted.
Stars are moving away from us…
v = Ho d
Ho - the Hubble constant
~ 70 km/s /Mparsec (Mpc ~ 3x106 lightyears)
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The nature of light
• Isaac Newton ~1670
– Light is a beam of particles (corpuscles)
• Christian Huygens 1678
– Light is composed of waves
• Thomas Young 1802
– Experiments proved wavelike nature of light
The debate continues…
• Maxwell ~ 1870
– Light is a form of electromagnetism
• Einstein 1905
– Photoelectric effect - light is quantized
– Newton’s corpuscles are called photons
• Current view
– Wave-particle duality: sometimes acts like a wave,
sometimes like a particle!
Waves
• Most waves we are familiar with propagate
(move) in a medium
– Sound waves move in air
– Ocean waves in water
– Stadium waves in spectators
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Waves
• Most waves we are familiar with propagate (move) in
a medium
– Sound waves move in air
– Ocean waves in water
– Stadium waves in spectators
Fields
• Charged particles (e.g. electron) create
electric fields
• Moving charged particles create
magnetic fields as well.
• But what about light?
– It travels through empty space…
Electromagnetic Waves
Fields can exert a force at a distance.
Gravity is a field.
Gravitational Waves
• A rapidly oscillating charge creates both
E and B fields
This creates a wave that can propagate.
http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=52
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Properties of Waves
Properties of Waves
frequency ν = 1/Τ units of Hz (s -1)
field
field
λ (wavelength)
Τ (period)
A (amplitude)
A (amplitude)
position
time
Velocity v = ν λ = c (for light)
http://www.udel.edu/idsardi/sinewave/sinewave.html
http://www.cbu. edu/~jvarrian/applets/waves1/lontra_g.htm
Adding waves
• Can get beat patterns and standing
waves.
http://www.kettering.edu/~drussell/Demos/superposition/superposition.html
What we see
• Amplitude = Intensity (brightness)
• Frequency -> Color
• Wavelength -> Color
λ= 6.5 X 10 -7 m
ν = 4.5 X 10 14 Hz
λ= 4.5 X 10 -7 m
ν = 7 X 10 14 Hz
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Unit Prefixes
•
•
•
•
•
•
•
•
p - pico n - nano µ - micro m - milli k - kilo M - mega G - giga T - tera -
10-12
10-9
10-6
10-3
103
106
109
1012
Examples:
1 nm = 10 -3 µm
1 ms = 10 6 ps
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