Download 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.