Chapter2: Light and EM Spectrum
Professor Mohammad Sajjad Alam
University at Albany
September 28, 2010
Adapted from Web
• 18.1 The Electromagnetic Spectrum
• 18.2 Interference, Diffraction, and
Polarization
• 18.3 Special Relativity
Chapter 18 Objectives
1.
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4.
5.
6.
Calculate the frequency or wavelength of light when given one
of the two.
Describe the relationship between frequency, energy, color, and
wavelength.
Identify at least three different waves of the electromagnetic
spectrum and an application of each.
Interpret the interference pattern from a diffraction grating.
Use the concept of polarization to explain what happens as light
passes through two polarizers.
Describe at least two implications of special relativity with
regards to energy, time, mass, or distance.
Chapter 18 Vocabulary Terms
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• x-ray
spectrum
microwave
index of refraction
electromagnetic
wave
spectrometer
gamma ray
radio wave
transmission axis
diffraction grating
special relativity
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polarization
polarizer rest
energy destructive
interference
ultraviolet
time dilation
infrared
speed of light
constructive
interference
 visible light
 wavelength
18.1 The Electromagnetic Spectrum
Key Question:
What is the electromagnetic
spectrum?
*Students read Section 18.1
BEFORE Investigation 18.1
18.1 The Electromagnetic Spectrum
• The energy field created
by electricity and
magnetism can oscillate
and it supports waves
that move.
• These waves are called
electromagnetic waves.
18.1 The Electromagnetic Spectrum
• Electromagnetic waves have
both an electric part and a
magnetic part and the two parts
exchange energy back and forth.
• A 3-D view of an
electromagnetic wave shows the
electric and magnetic portions.
 The wavelength and amplitude of the waves are
labeled λ and A, respectively.
18.1 The Electromagnetic Spectrum
• The higher the frequency of the light, the higher the energy of
the wave.
• Since color is related to energy, there is also a direct relation
between color, frequency, and wavelength.
18.1 Speed of Light
Speed of light
3 x 108 m/sec
c = f l
Wavelength (m)
Frequency (Hz)
18.1 Calculate wavelength
• Calculate the wavelength
in air of blue-green light
that has a frequency of
600 × 1012 Hz.
18.1 Waves of the electromagnetic
spectrum
• Visible light is a small part of the energy range of electromagnetic waves.
• The whole range is called the electromagnetic spectrum and visible light is
in the middle of it.
18.1 Waves of the electromagnetic
spectrum
– Radio waves are on the lowfrequency end of the
spectrum.
– Microwaves range in length
from approximately 30 cm
(about 12 inches) to about 1
mm.
– The infrared (or IR) region of
the electromagnetic spectrum
lies between microwaves and
visible light.
18.1 Waves of the electromagnetic
spectrum
– Ultraviolet radiation has a
range of wavelengths from
400 down to about 10 nm.
– X-rays are high-frequency
waves that have great
penetrating power and are
used extensively in medical
and manufacturing
applications.
– Gamma rays are generated in
nuclear reactions.
18.2 Interference, Diffraction, and
Polarization
Key Question:
What are some ways
light behaves like a
wave?
*Students read Section 18.2
AFTER Investigation 18.2
•
18.2 Interference, Diffraction, and
In 1807, Thomas Young
Polarization
(1773-1829) did the most
convincing experiment
demonstrating that light is a
wave.
• A beam of light fell on a pair
of parallel, very thin slits in
a piece of metal.
• After passing through the
• A pattern of alternating
slits, the light fell on a
bright and dark bands
screen.
formed is called an
interference pattern.
18.2 Diffraction gratings
• A diffraction grating is a precise array of tiny engraved lines,
each of which allows light through.
• The spectrum produced is a mixture of many different
wavelengths of light.
18.2 How a Diffraction Grating Works
When you look at a diffracted light
you see:
– the light straight ahead as if
the grating were transparent.
– a "central bright spot".
– the interference of all other
light waves from many
different grooves produces a
scattered pattern called a
spectrum.
18.2 Spectrometer
• A spectrometer is a device
that measures the
wavelength of light.
• A diffraction grating can be
used to make a spectrometer
because the wavelength of
the light at the first-order
bright spot can be expressed
in a mathematical
relationship.
18.2 Grating Formula
distance between grating lines (m)
wavelength
of light (nm)
l = d sinq ~ dw
L
distance
between
2 first order
bright spots
distance
between
screen and glasses
d= 13,500 lines/inch = ? lines/m
18.2 Polarization
• Polarization is another wave property of light.
• The fact that light shows polarization tells us that light is a
transverse wave.
18.2 Polarization
• Polarization is a vector.
• A wave with polarization at
45 degrees can be
represented as the sum of
two waves.
• Each of the component
waves has smaller
amplitude.
18.2 Polarization
• A polarizer is a material that selectively absorbs light depending on
polarization.
• A polarizer re-emits a fraction of incident light polarized at an angle to the
transmission axis.
18.2 Applications of Polarizers
• Polarizing sunglasses are used to
reduce the glare of reflected
light
• The LCD (liquid crystal diode)
screen on a laptop computer
uses polarized light to make
pictures.