Chapter 23: The Physical Nature of Light
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Transcript Chapter 23: The Physical Nature of Light
Light and Optics
Unit 8: Light and Optics
Chapter 23: The Physical Nature
of Light
23.1
Electromagnetic Spectrum
23.2
Interference, Diffraction, and
Polarization
23.3
The Dual Nature of Light
23.1 Investigation: The Electromagnetic
Spectrum
Key Question:
What is the electromagnetic spectrum?
Objectives:
Research one type of wave that is part of the electromagnetic
spectrum.
Prepare a poster containing the information they discover.
Make an oral presentation to share their findings with the class.
Electromagnetic waves
If
you could shake the
magnet up and down 450
trillion times per second, you
would make waves of red
light with a frequency of
about 450 THz.
Light
and radio waves are
waves of electromagnetism.
Electromagnetic waves
If
you switch electricity on
and off repeatedly, the
oscillating electricity makes
an electromagnetic wave.
This
is exactly how radio
towers make radio waves.
The electromagnetic spectrum
Light,
The
like sound and heat, is a form of energy.
visible light we see is part of the
electromagnetic spectrum.
Electromagnetic spectrum
The
entire range of electromagnetic waves,
including all possible frequencies, is called the
electromagnetic spectrum.
This
spectrum includes both visible light and
invisible waves:
—
radio wave
— microwaves
— infrared light
— ultraviolet light
— X-rays
— gamma rays
Wavelength and
Frequency of Light
Because
the wavelength of
light is so small, scientists
measure it in nanometers.
One
nanometer (nm) is one
billionth of a meter
(0.000000001 m).
Frequencies and wavelengths of light
Since
color is related to energy, there is a direct
relationship between color (energy) and frequency
and an inverse relationship between color (energy)
and wavelength.
Electromagnetic wave speed
All
electromagnetic waves travel at the same speed
in a vacuum, the speed of light—3 × 108 m/s.
When
moving through a material, the frequency of
light stays the same.
Index of refraction
The
index of refraction (n)
for a material is the ratio of
the speed of light in a
vacuum to the speed of
light in that material.
Low-energy electromagnetic waves
We
classify the energy of electromagnetic waves by
comparing it to the energy it takes to remove an
electron from an atom.
Low
energy waves, like visible light, do not have
enough energy to break most chemical bonds:
—
—
—
Radio waves are the lowest-frequency waves.
Microwaves range in length from 1 mm to 30 cm.
Infrared waves include wavelengths from 1 mm to about
700 nm.
High-energy electromagnetic waves
Ultraviolet
light has enough energy to remove
electrons and to break chemical bonds.
X-rays
are high-frequency waves that are used
extensively in medical and manufacturing
applications.
Gamma
rays are generated in nuclear reactions,
and can strip the innermost electrons out of an atom.
Unit 8: Light and Optics
Chapter 23: The Physical Nature
of Light
23.1
Electromagnetic Spectrum
23.2
Interference, Diffraction, and
Polarization
23.3
The Dual Nature of Light
23.2 Investigation: The Wave Nature of Light:
Polarization
Key Question:
What are some ways light behaves
like a wave?
Objectives:
Use a string to demonstrate the behavior of a light wave.
Explain the interaction of polarizers, using the wave theory of
light.
Diffraction and shadows
Like
sound and water waves,
light shows interference,
diffraction and polarization.
Diffraction
occurs when a wave
passes through an opening not
too much wider than the
wavelength of the wave.
Observing
diffraction with light
is evidence that light is a wave.
Diffraction and shadows
You
can see diffraction in
a shadow cast by a sharp
edge with light from a
laser.
The
edge of the shadow
has ripples in it.
The
ripples are caused
by diffraction.
Young’s double slit experiment
In
1807, Thomas Young proved light was a wave
when he showed that two beams of light could
interfere with each other.
Light is a wave
The
bright bands in an
interference pattern are
where the light waves
from both slits are in
phase at the screen
(constructive
interference).
The
dark bands appear
where the light waves
reach the screen out of
phase (destructive
interference).
Diffraction gratings
A diffraction
grating actually a series of thin parallel
grooves on a piece of glass or plastic.
When
light goes through a diffraction grating, each
groove scatters the light so the grating acts like many
parallel slits.
Spectrometers
A spectrometer
is a calibrated diffraction grating
used to create a spectrum.
The
spectrometer has a scale that allows you to read
different wavelengths of light directly from the pattern
of light made by the grating.
Polarization
The
orientation of light is called its polarization.
Only
transverse waves can have polarization.
Polarizers
A polarizer
is a material
that allows light of only
one polarization to pass
through it.
Light
with a single
polarization is called
polarized light.
Applications of
polarization
Polarized
sunglasses
reduce glare because
they selectively absorb
light with horizontal
polarization while letting
other light through.
Applications of polarization
Images
on a LCD (liquid crystal display)are made
using polarized light.
Each
liquid crystal window can be electronically
controlled to act like a polarizer, or not.
Unit 8: Light and Optics
Chapter 23: The Physical Nature
of Light
23.1
Electromagnetic Spectrum
23.2
Interference, Diffraction, and
Polarization
23.3
The Dual Nature of Light
23.3 Investigation: The Particle Nature of Light:
Phosphorescence
Key Question:
How does light fit into the atomic theory of matter?
Objectives:
Explore the quantum theory of light.
Experiment with a photoluminescent material
Energy, color and light
The
lowest-energy
photons we can see are
the ones that appear red
to our eyes.
White
light is a mixture of
photons with a range of
energy.
Energy and intensity of light
The
intensity of light is a
combination of both the
number of photons and the
energy per photon.
To
make a red light with an
intensity of 100 W/m2 takes
a lot more photons than it
does to make the same
intensity with blue light.
Energy and intensity of light
If
glow-in-the-dark plastic is
exposed to light, it stores
some energy and releases
the energy later by giving
off light.
The
process of releasing
stored light energy is called
photoluminescence.
Glow-in-the-dark plastic
demonstrates that a single
atom only absorbs a
single photon at a time.
Light and atoms
Almost
all atoms absorb
and emit light.
For
most atoms, the
absorption and emission
of light happens in less
than one-millionth of a
second.
How 3-D Movies Work
Cinematographers,
ophthalmologists, optical
engineers, and computer
graphic designers all play a role
in the development of modern 3D movie technology.
To create the illusion of threedimensions on a flat screen,
each eye must receive its own
separate image of the movie,
from a slightly different
perspective, mimicking the way
your eyes take in a real threedimensional scene.