Transcript The physics of light

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Pass/Fail on
Tuesday 6:00 pm!!
Question # 1: answer A
Setup: Question # 2: answer B
Question # 3: answer E
Reading assignment
Chapter 5 (Light), pp. 137-164
Please take a moment to mute your cell phone!
The physics of light
Light … is an electromagnetic wave:
• … is a wave pattern
of electric and magnetic fields
• … propagates in vacuum
• … has wavelength (blue: 400 nm, red: 700 nm)
0.4 mm
0.7 mm
Learn
units of distance!
1 mm = millimeter
Millimeter
(size of ant)
1000 mm = 1 mm
Micrometer
(size of a germ)
1000 nm = 1 mm
Nanometer
(size of ten atoms)
Speed of light c = 300,000 km/s: Discovered by: Olaus Roemer (1675)
Watch
07JupiterMoonDance.mov
Light takes
• a few minutes/hours
to move around in
the solar system
• years or more
to go from
star to star
• Eclipses of Jupiter’s moons come a few minutes late
when Jupiter is on the far side of its orbit.
• Reason: light needs extra time to reach us.
• Conclusion: light needs 9 minutes to traverse the Sun-Earth distance.
Olaf Römer
The electromagnetic spectrum
Light has a wavelength: l
Wavelength of light means color:
• Blue has short wavelength
• Red has long wavelength
What is
beyond blue?
These are colors of light
invisible to the eye.
Bluer than blue is UV (ultraviolet) - even shorter wavelength
Redder than red is IR (infrared) - even longer wavelength
And what is beyond those wavelengths?
(A prism breaks up white light into colors.)
The full spectrum
Prism
Radio | Infrared | Visible
Radio antenna
Slit
White
light
| Ultraviolet X-ray| g -ray
Eye / Telescope
Need spacecraft
(Far UV, X-ray, g-ray)
Need spacecraft
They all move with the same speed c = 300,000 km/s
Questions coming …
Question 4
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Which one is moving fastest?
A Radio waves
B X-rays
C Visible light
D Laser
E The same
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Question 5
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What is the difference between red and blue light?
A Intensity.
B Speed.
C Wavelength.
D Blue is composite, red is not.
E Red is existence of light, blue is lack of light.
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Next question coming …
Question 6
How many of the following types of “light”
penetrates the atmosphere?
• Radio waves
• Far IR
• Regular light
• Far UV
• X-rays
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4321087659
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A1 B 2 C3 D4 E5
Next question coming …
Question 7
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How was the speed of light measured for the first time?
sec
A In an experiment with a laser beam.
B By the time delay the astronauts’ answer came from the Moon.
C Observing the moons of Jupiter.
D By measuring the radar signal reflected by the Moon.
E By measuring the time light takes to arrive to us
from another galaxy.
Recall:
Structure of
 Chemistry cannot change the nucleus, only bind atoms into molecules
atomchemical properties.
 Isotopes: same number of p and e, different n. Same
Examples:
H:
Deutron:
He:
1p
1p
2p
0n
1n
2n
1e
1 e (same as H but heavier)
2e
Photons
Einstein’s
discovery:
Red l = 720 nm, E = 1.7 eV
Blue l = 420 nm, E = 3 eV
UV l = 100 nm, E = 12 eV
X-ray l = 1 nm, E = 1200 eV
 light comes in chunks (photons)
 each photon has E=1240/l energy
in electron-volts (eV)
in nm’s
Enough to destroy most atoms
Destroys everything if strong
Turn in your scantron now
personally!