Transcript Light: The Cosmic Messenger

Light: The Cosmic Messenger
What is light?
What is light?
• Does the prism
PRODUCE colors,
or
were they there
already in the light?
• How can you tell?
Observation: Color is property of light!
White light is
composed of
different colors when
shone through
glass….
…but the glass is not
creating those
colors!
Observation: Light is FAST!
Galileo tried to measure with lanterns!
…but light was too quick!
1 mile there & back in
0.00001 seconds!
Observation: Light is FAST!
Roemer tried to measure it with
Jupiter’s Moons!
…by timing when
they passed in
front or behind
Jupiter
Observation: Light is FAST!
Hubble
Space
Telescope
image of
Ganymede
being
eclipsed by
Jupiter
More Observations: Speed of Light?
More Observations: Speed of Light?
Speed of Light is not infinite!
Roemer’s observations of Jupiter’s Moon’s Eclipses
demonstrated light moves at a finite speed
Even More Observations:
Light has “energy”
Non-visible light
(beyond the red end
of the spectrum)
has energy, too!
Observation: Water Waves naturally
interfere & create noticeable patterns
Observation: Light has a wavelike
property, too!
Young’s Experiment (1801)
Wavelength and Frequency
wavelength  frequency = speed of a wave
Wavelength of light?
Microwaves on buttered bread!??
See: The Naked Scientists Podcast
The entire EM
Spectrum
What we “see” is
only a small part of
what there is!
EM Spectrum
Varies by…
Size (wavelength, color)
Energy
How the waves are
detected
But not….
How fast they move
through space!
Atmospheric “Windows” to the stars & universe:
Visible & Radio light
Observations of Nature
• Electricity acts through space over a distance
– Lightening, sparks on a doorknob
Observations of Nature
Magnetism acts through space over a distance
– Two magnets attract or repel one another without
touching
More Observations
• If you spin a conductor in a
magnetic field, you get
electricity!
– Electric Generators
– Portable gas generators
More Observations
• If you run electricity into a coil,
you get a magnet!
– “Electromagnetic” cranes
– Auto solenoids
– Electric Motors
Maxwell’s Observations
• Change Electricity => create magnetism
• Change Magnetism => create electricity
• Continuously change both, continuously create
radiation!
• Radiation created moves at “c” – the speed of
light!
The Wave Model of Light
Electro-magnetic radiation!
Light is an
electromagnetic wave.
The Electromagnetic Spectrum
Still More Observations of Nature
• Einstein’s Photo-electric Effect showed light can
eject individual electrons
– Energy depended upon light’s color!
Single electrons
emerge
Light shining
onto metal
Metal plate
Still More Observations of Nature
• Planck’s Energy Curve showed light can be modeled with
specific “quanta” of energy
– Peak depended upon light’s color!
Metal plate
The Particle Model of Light
• Particles of light are called
photons.
• Each photon has
wavelength & frequency.
• Energy of a photon
depends on frequency.
Wavelength, Frequency, & Energy
l  f = c
l = wavelength, f = frequency
c = 3.00  108 m/s = speed of light
E = h  f = photon energy
h = 6.626  10−34 joule  s
Planck’s constant
Thought Question
Which is correct?
The higher the photon energy,
A. the longer its wavelength.
B. the shorter its wavelength.
C. Energy is independent of wavelength.
Thought Question
The higher the photon energy,
B.
the shorter its wavelength.
Which explains why…
•
•
X-rays can kill you!
Country-western music on the radio can’t