Download Light

Properties
of
Light
Chapter 27
Models of Light
A.Socrates and Plato believed
"streamers" or antennas were
emitted from the eyes.
B.Euclid and Descartes had similar
theories.
C.Greeks believed anything that
could be seen emitted light
waves.
Models of Light
D. Newton introduced the particle
theory. He said that light travels in
straight lines unlike waves.
Models of Light
E. Huygen disagreed with Newton and
said that light is a wave because it
spreads out sometimes.
Models of Light
F. Einstein realized that light acts not
only as waves but as quantum particles
now known as photons.
G. Light has dual properties – Particles
and Waves
Question: Does light have mass?
Black Holes
• Once was a star at least twice the mass of the
sun that exhausted its fuel and collapsed to
infinite density
• An event horizon exists
– inside which the intense gravitational field of
the black hole prevents any type of
electromagnetic radiation including light from
escaping
A ravenous black hole
This is a Hubble Space Telescope image of an
800-light-year-wide spiral-shaped disk of dust
fueling a massive black hole in the center of
galaxy NGC 4261, located 100 million lightyears away in the direction of the
constellation Virgo. By measuring the speed
of gas swirling around the black hole,
astronomers calculate that the object at the
center of the disk is 1.2 billion times the mass
of our sun.
Speed of Light
• A. Roemer was the
first person to try to
measure the speed
of light in
observations of Io.
• His calculation was
300,000 km/s.
Speed of Light
• In 1880, Albert Michelson measured the speed of
light to be 299,920 km/s.
–Used spinning octagonal mirrors and a mirror far away
–Adjusted spinning speed to calculate speed of light
In 1907, he won the Nobel prize in physics.
Electromagnetic Wave Velocity
Light could go around the world 7.5 times each second
http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/geo-high.html
Electromagnetic Wave Velocity
Light takes 8 minutes to travel from the sun to earth.
Electromagnetic Wave Velocity
The next closest star is 4 light years away
The diameter of our galaxy is 100,000 light years
Some galaxies are 10 billion light years away
http://bluepoint.egenet.net/sagan/galaxy.gif
Electromagnetic Spectrum
•
•
•
•
Energy moving at speed of light
Vary by f and l only
Light is only a small part
Below light is infrared, above is ultraviolet
Electromagnetic Spectrum
Radio Waves – communication TV and Radio
Microwaves –cooking & cell phones
Infrared – “heat waves”
Visible Light – detected by your eyes
Ultraviolet – causes sunburns
X-rays – penetrates tissue
Gamma Rays – most energetic
The Visible Spectrum
•A range of light waves extending in wavelength
from about 400 to 700 nanometers
• Less than 1% of the entire spectrum
Questions
• Is it correct to say that radio wave is
a low-frequency light wave?
• Is a radio wave also a sound wave?
Transparent Materials
Transparent - the term applied to materials
through which light can pass in straight lines
Visible Light and Glass
Visible light maintains the same frequency
when it enters glass
But the velocity changes:
1. Light travels 0.75x the original speed
in water
2. In glass – 0.67x.
3. In diamond – 0.41x
When the light comes out it regains its speed
Opaque Materials
Opaque - the term applied to materials that
absorb EM
Opaque Materials
Opaque materials absorb EM
– ex.) wood, metal, rocks ,etc.
The light energy is transformed into
random kinetic energy
– makes the object warmer because
it absorbs the electromagnetic energy
http://www.ch.ic.ac.uk/local/projects/steer/chloro.htm
Example Questions
• Are clouds transparent or opaque to
visible light?
–Answer: opaque
• Are clouds transparent or opaque to
ultraviolet light?
–Answer: transparent
Example Questions
Are windows transparent or opaque to
visible light?
–Answer: transparent
Are windows transparent or opaque to
ultraviolet light?
–Answer: opaque
–Due to Law of Conservation of
Energy the EM is given off as heat
Shadows
Umbra - the darker part of a shadow where
all the light is blocked
Penumbra - a partial shadow
These terms also apply to Solar Eclipses and
Lunar Eclipses.
Solar Eclipse
Umbra
Penumbra
• A solar eclipse occurs when
the Moon passes in front of
the Sun.
A lunar eclipse occurs when
the Moon passes into the
Earth's shadow.
Questions
• Which type of eclipse is dangerous
to view with the unprotected eye?
• Why are lunar eclipses more
commonly seen than solar eclipses?
Seeing the Light – The Eye
• Cornea - does most of the focusing
• Iris - has the eye color and controls light intensity
• Pupil - the hole in the eye
• Lens - does remainder of focusing
• Retina - location of light sensors, has rods and cones
• Fovea - center of vision, predominantly cones
• Blind spot - optic nerve exit, no light sensors
Parts of the Eye
Detectors on the Fovea
–Rods
• light intensity and motion sensitive
–Cones
• color sensitive
The blind spot for the eye is cause by
the optic nerve.
p. 474, Conceptual Physics 2002
Color Vision!
Colorblindness - about 10% of the
population
• Red-green is predominant
• Yellow-blue - a few
• Total – some
Mostly males – X-linked trait
Color Deficiency
Optical Illusions
Optical Illusions
Optical Illusions
Are the horizontal lines parallel, or do they slope?
Optical Illusions
Optical Illusions
During the Optical Art (OpArt) Movement
of the 1960s, artists would create all sort of
puzzling effects with color.
This "flashing squares" drawing seems to
wobble and flash when you concentrate on
one particular area of the image.
How many squares can you see in this
diagram?
Can you feel the "motion" of the image?
Optical Illusions
Optical Illusions
Can you count the black
dots?
Optical Illusions
Myopia (Near-Sightedness)
People with near-sightedness
cannot see clearly at distance.
Hyperopia (Farsightedness)
People with far-sightedness
cannot see clearly up close
You will observe a total eclipse
of the sun when...
(a) you’re in the penumbra of the moon’s shadow
(b) you’re in the umbra of the moon’s shadow
(c) sunlight diffracts around the moon
(d) sunlight reflects from the moon to the earth
The speed of light...
(a) has never been measured
(b) is about the same as that of
sound
(c) is infinitely fast
(d) is very fast, but not infinite
In the dark in late evening no
color is seen because of lack of
stimulation of
a. rods.
b. cones.
c. cornea.
d. crystalline lens.
Self Test
On a Blank Page....
• List the Seven Forms of Light
– Which has the longest wavelength?
– Which has the highest frequency
– Which has the highest energy?
– Which has the fastest speed?
Structure of
the Atom
Electron
Energy Levels
or Orbits
Neutron
Proton
Ground State of Electron
• Energy level it
normally
occupies
– state of lowest
energy for that
electron
Excited State of Electron
• Temporarily energy
state greater than its
ground state
• e- can become excited if
it is given extra energy
– absorbs a photon, or
packet of light
– or collides with a nearby
atom or particle
Emission of Photon
• Electrons do not stay
in excited states for
very long
– they soon return to
their ground states
– emitting a photon
with the same
energy as the one
that was absorbed
Transitions among the various orbitals are unique for
each element because the energy levels are uniquely
determined by the protons and neutrons in the nucleus
• When the electrons of a certain atom
return to lower orbitals from excited
states, the photons they emit have
energies that are characteristic of that
kind of atom
• This gives each element a unique
fingerprint, making it possible to
identify the elements present in a
container of gas, or even a star
Kirchhoff-Bunsen Experiment
These two scientists found that burning chemicals
over an open flame resulted in an emission spectrum
with bright lines
Emission Spectrum
They found that each chemical element produced its
own characteristic pattern of bright spectral lines
Hydrogen
Helium
Oxygen
Carbon
Every element can be “fingerprinted” by it spectra.
Incandescence
Hot, dense solids produce a continuous
spectrum.
Continuous Spectrum
The color of light emitted by a hot
object changes with its temperature
hottest glowing object  Bluish
White
Yellowish
Orange-ish
coolest glowing object  Reddish
Absorption Spectra
• Cool gas in front of a continuous source of light
produces an absorption line spectrum.
• Fraunhofer lines in our Sun's spectrum showed
that cool helium gas surrounds the Sun.
Absorption Spectrum
Matching Questions
Type of Spectrum
1. Emission Spectra
Appearance
a. All Colors
2. Continuous Spectra
b. Dark Lines
3. Absorption Spectra
c. Bright Lines
Matching Questions
1. Emission Spectra
a. Hot Solids
2. Continuous Spectra
b. Hot Stars
3. Absorption Spectra
c. Hot Gases
Fluorescence
• Some materials that are excited by
ultraviolet light can emit visible light
upon de-excitation
– The material’s electrons use some of the
energy during vibration
– The emitted light (bluish white) is at a
lower frequency (energy) than the UV
Fluorescent Lamps
• Primary excitation - electron collisions
with low pressure mercury vapor, and
ultraviolet light is given off
Secondary excitation - ultraviolet light is
absorbed by phosphors and these emit
visible light
Phosphorescence
• Phosphorescence - a type of light emission
that is the same as fluorescence except for a
delay between excitation and de-excitation.
• Electrons get "stuck" in an excited state and
de-excite gradually.
Lasers
• Light Amplification by Stimulated Emission of
Radiation
• Lasers produce coherent light.
– all the light waves have the frequency, phase
and direction.
•
Demo - Laser and chalk dust
Laser Type
Wavelength (nm)
Argon fluoride (UV)
193
Krypton fluoride (UV)
248
Nitrogen (UV)
337
Argon (blue)
488
Argon (green)
514
Helium neon (green)
543
Helium neon (red)
633
Rhodamine 6G dye (tunable)
570-650
Ruby (CrAlO3) (red)
694
Nd:Yag (NIR)
1064
Carbon dioxide (FIR)
10600
Matching Questions
1. Incandescence
a. mercury vapor
light tubes
2. Fluorescence
b. glow-in-the-dark
paints and plastics
3. Phosphorescence
c. light bulbs
with filaments
Matching Questions
1.Fluorescent Lamp
2.Incandescent Bulb
3. Laser
a. color changes
with temperature
b. only one color
of light
c. converts ultraviolet
light to visible light
Chapter 27
Different wavelengths of light are
perceived as different colors
Pure Colors: ROY G. BIV
White light contains equal amounts
of these colors (ROYGBIV)
Selective Reflection and
Transmission
At an interface, light can be...
–absorbed
–reflected
–transmitted
Fill in the Blanks
absorb all colors of light.
Black objects _______
reflect all colors of light.
White objects _______
transmit all colors of
Transparent objects _______
light
Color Filters
Red filters transmit red light and absorb
the other colors, etc.
Red objects reflect red light and absorb
the rest, etc.
In red light, what color do the red petals
and green leaves of a rose appear?
Answer: Petal appear red
Leaves appear to be black.
Mixing Colored Light
Color Addition
• Additive Primary Colors of Light:
• Red
• Green
• Blue
• One can produce any color by varying
amplitude and mixture or red, green, and
blue light.
Color Addition Example
Tiny dots called pixels on
Color TV's and Computer
Monitors are colored only red,
green, or blue
Mixing Colored Pigments Color Subtraction
Subtractive Primary Colors:
• Yellow
• Magenta
• Cyan
One can produce any color by varying
the amount of yellow, magenta and cyan
pigments.
Green Pigment
Absorbed =
Red Pigment
Colors of Light
Yellow Pigment
Cyan Pigment
+
+
Absorbed =
Yellow + Cyan = Green Pigment
+
+
Absorbed =
In-Class Exercise
1. Make a list of the seven forms of light
in order of decreasing wavelength.
2. Draw a Color Addition Diagram using
overlapping colored spotlights. Label
all 7 colors.
3. Draw a Color Subtraction Diagram
using overlapping paints. Label all 7
colors.
Colors of Light
magenta
red
yellow
white
green
blue
cyan
yellow
green
red
black
magenta
cyan
blue
Why is the Sky Blue?
Nitrogen and Oxygen
in our atmosphere
scatter high frequencies of light
Why is the Ocean Greenish Blue?
Red light is
absorbed
by the
molecules
in the
water
How have
fish
adapted?
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?
• Clouds are composed of water drople
• Different-sized droplets create differ
types of scattered frequencies
– Small droplets scatter blue
– Bigger scatter higher frequencies (like gree
– Even bigger droplets scatter red
• This creates a cloud that appears wh
Complementary Light Colors any two colors that add together
to produce white
e.g. magenta + green = white
After Images - Conal Fatigue
• The human eye will see complimentary
colors after staring at a color picture.
• Demos:
• Colored Shapes
• Texas Flag
• American Flag
• Rose
• Lincoln
If you look at the yellow Sun just
before sunset for a few seconds and
then look at a white cloud you see
_______ afterimages of the Sun.
a) red
b) green
 c) blue
d) cyan
A mixture of cyan and yellow paints
gives __________ paint.
 a) green
b) red
c) black
d) white
e) blue
Adding red and green light
gives us what color?
 a) yellow
b) cyan
c) blue
d) magenta
Suppose that two flashlight beams are shone on
a white screen, one through a pane of blue glass
and the other through a pane of green glass.
What color appears on the screen where the two
beams overlap?
a) yellow
b) green
c) cyan
d) magenta
e) red
If a car headlight emitted only
yellow light, the normally green
grass appears to be
 a) green.
b) red.
c) black.
d) white.
e) yellow.
To see an after image of a red,
white and blue Texas flag one
would first stare at a flag with the
complimentary colors
a) yellow, violet, and green.
b) cyan, yellow, and magenta.
 c) cyan, black, and yellow.
d) yellow, cyan and green.
The worst thing that you can do for the
health of a green-leafed plant is to
illuminate it with only
a) red light.
 b) green light.
c) blue light.
d) all are equally bad.
e) none of these.
Most of the light that we see
has undergone
(a) selective interference
(b) selective transmission
(c) selective reflection
(d) selective refraction
A mixture of magenta and green
lights give white light. These
two colors are
(a) additive primaries
(b) secondary colors
(c) complementary colors
(d) fluorescent colors
(e) interference colors
Mixing yellow paint and
magenta paint gives what
color?
 (a) red
(b) green
(c) blue
(d) cyan
What color would red cloth
appear if it were illuminated by
cyan light?
(a) cyan
(b) red
(c) yellow
(d) green
 (e) black