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Transcript
Light
The Electromagnetic
Connection
• A changing magnetic field produces an electric
field, and a changing electric field produces a
magnetic field.
• Electric and Magnetic fields can produce forces
on charges
• An accelerating charge produces
electromagnetic waves (radiation)
• Both electric and magnetic fields can
transport energy
– Electric field energy used in electrical circuits,
e.g., released in lightning
– Magnetic field carries energy through
transformer, for example
Electromagnetic Radiation
• Interrelated electric and magnetic fields traveling
through space
• All electromagnetic radiation travels at c = 3108
m/s in vacuum – the cosmic speed limit!
– real number is 299792458.0 m/s exactly
Examples of Electromagnetic
Radiation
• AM and FM radio waves (including TV
signals)
• Cell phone communication links
• Microwaves
• Infrared radiation
• Light
• X-rays
• Gamma rays
• What distinguishes these from one
•
Uses of Electromagnetic
Waves
Communication systems
– One-way and two-way
•
•
•
•
•
Radar
Cooking (with microwaves)
Medical Imaging (X rays)
“Night Vision” (infrared)
Astronomy (radio, wave, IR, visible, UV,
gamma)
All that we experience through our eyes is conveyed by
electromagnetic radiation…
The Electromagnetic Spectrum
• Relationship between frequency, speed
and wavelength
f ·l = c
f is frequency, l is wavelength, c is
speed of light
• Different frequencies of electromagnetic
radiation are better suited to different
purposes
• The frequency of a radio wave determines its
propagation characteristics through various
media
Polarization of Radio
Waves
Transmitting
antenna
E
Reception of Radio Waves
E
Receiving antenna works best
when ‘tuned’ to the
wavelength of the signal, and
has proper polarization
Electrons in antenna are “jiggled”
by passage of electromagnetic wave
Questions
Why are car radio antennas vertical?
Why are cell phone antennas so short?
How do polarizing sunglasses work?
Radio waves
• Radio waves –
between 100-1m long
• Communication, radio
and tv
• AM- amplitude
modulation
• FM Frequency
modulation
Microwaves
• Wavelength between
1m and 1mm long
• Used for
communication, radar
and cooking food
Infrared
• Wavelength between
just below visible light
• Between 750nm and
1mm
• Perceived as heat
• Night vision, heating
• Nanometer is
1/1000000000 of a
meter
Visible light
• Color
Red
Orange
Yellow
Green
Blue
Violet
Wavelength(nm)
780 - 622
622 - 597
597 - 577
577 - 492
492 - 455
455 - 390
Frequency
384 - 482
482 - 503
503 - 520
520 - 610
610 - 659
659 - 769
Spectroscopy
• spectrometer is an optical instrument
used to measure properties of light over a
specific portion of the electromagnetic
spectrum
• Elements give off different emission
spectra when they glow
• These can be used to identify the
presence of elements
Hydrogen
Helium
Carbon
Nitrogen
Oxygen
Neon
Sodium
Properties of visible light
• Absorption- light is
absorbed by matter
• Why does your
flashlight become dim
at a distance
• Scattering is the
release of light by the
matter
• Prism- Refracts light
twice, different
wavelenghts are
refracted more than
others
The Visible Spectrum
A range of light waves extending in
wavelength from about 400 to 700
nanometers.
Things that create a Spectrum
•
•
•
•
Prism
Raindrops
CD’s
Spectroscopes
– Contains a Diffraction Grating
Emission Spectra
• Hot gas produces a bright line emission spectrum.
(Bright Lines)
Absorption
• When an electron is raised to a higher
energy level, the atom is said to be
excited.
Emission
• When the electron returns to a lower
energy level, energy is released in the
form of light.
Transmission
• Matter that allows visible light to easily
pass through are transparent
• Matter that scatters light is translucent
• Matter that does not transmit light is called
opaque
Ultraviolet
• Wavelenghts above
visible light 400nm1nm
• Photons have enough
energy to kill living
cells
• Present in sunlight
X-rays
• 10 to 0.01
nanometers,
• Diagnostic imaging
• Astronomy
Gamma rays
• Shorter then .01nm
• Radioactive decay
• Great penetrating
power
Parts of the Human Eye
(Supplement Handout)
•
•
•
•
•
•
•
Cornea – bends light
Iris – controls the amount of light
Pupil - opening
Lens – focuses light onto retina
Retina – back of eye
Fovea – center of your vision
Optic Nerve – “signal wire” (causes blind spot)
Blind Spot Demo
O
X
• Rods - brightness receptors
• Cones - color receptors
– Three Types: Red, Green, Blue
• Cones are more numerous in the center of
your vision.
• Rods are more numerous around the
periphery of your vision.
• Demo – Moving markers near periphery
Color Deficiency
Color Vision
• Colorblindness - about 10% of population
• Red-green is predominant
• Yellow-blue - a few
What is color?
• Different wavelengths of light are
perceived as different colors.
• White light contains equal amounts of
these colors. (ROY G. BIV)
Fill in the Blanks
• Black objects _______ all of the pure
absorb
colors.
• White objects _______ all of the pure
reflect
colors.
• Transparent objects _______ all of the
transmit
pure colors.
Mixing Colored Light
Color Addition
• Additive Primary Colors:
• Red
• Green
• Blue
• One can produce any color by varying
amplitude and mixture or red, green and
blue light.
• Complementary Colors - any two colors
that add together to produce white
• e.g. magenta + green = white
After Images and “Conal Fatigue”
• The human eye will see complimentary
colors after staring at a color picture.
Transparent and Opaque
• Transparent
– you can identify objects through it
• Translucent
– you see diffuse light coming through it
• Opaque
– you cannot see any light coming through it
– Opaque objects cast shadows.
The Electromagnetic
Spectrum
•
•
•
•
•
•
•
Radio Waves
Microwaves
Infrared
Visible Light
Ultraviolet
X-rays
Gamma Rays
mnemonic
•
•
•
•
•
•
•
Raging
Martians
Invade
Roy G. Biv
Using
X-rays and
Gamma Rays
Atmospheric Refraction
• Our atmosphere can bend light
and create distorted images
called mirages.
•
http://astro.sfasu.edu/movies/Highway Mirage 1.mpg
• What causes stars to twinkle?
– Atmospheric Turbulence
Because of atmospheric refraction,
we have lingering, elliptical sunsets.
Sun
Sun
Earth
Holography
• Holography- technique that produces a 3D image without a lens
Why is the sky blue?
• Nitrogen and Oxygen in our atmosphere
scatter high frequencies of light.
Why are sunsets red?
• Red light is scattered the least by our
atmosphere.
• The greatest path of sunlight through the
atmosphere is at sunset or sunrise.
Why are clouds white?
• The color of light scattered by
clusters of water molecules vary
with the size of the clusters.
• The size of clusters of water
molecules (droplets) vary in clouds.
What happens when the source in in motion?
Movie #1
Java
Long Wavelength
Low Frequency
Low Pitch
Ooooooo!!!
Short Wavelength
High Frequency
High Pitch
Weeeeeeeee!!!
Source in Motion
As a team, describe
what happens to the
wavelength, frequency,
and pitch both in front of
and behind a moving
sound source.
Doppler Effect
• Examples:
– moving cars and trains
• Sound
• Movie #2
– moving buzzer in a nerf ball (in class)