Unit 6, Chapter 18

1
Unit 6, Chapter 18
CPO Science Foundations of Physics
2
Unit 6 Light and Optics
Chapter 18 Wave Properties of Light

• 18.1 The Electromagnetic Spectrum
• 18.2 Interference, Diffraction, and
  Polarization
• 18.3 Special Relativity

3
Chapter 18 Objectives

• 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.

4
Chapter 18 Vocabulary Terms

• x-ray
• spectrum
• microwave
• index of refraction
• electromagnetic wave
• spectrometer
• gamma ray
• radio wave
• transmission axis
• diffraction grating
• special relativity

• polarization
• polarizer rest
• energy destructive
• interference
• ultraviolet
• time dilation
• infrared
• speed of light
• constructive interference
• visible light
• wavelength

5
18.1 The Electromagnetic Spectrum

• Key Question
• What is the electromagnetic spectrum?

Students read Section 18.1 BEFORE Investigation
18.1
6
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.

7
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 ? and A, respectively.

8
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.

9
18.1 Speed of Light
Wavelength (m)
c f l
Speed of light 3 x 108 m/sec
Frequency (Hz)
10
18.1 Calculate wavelength

• Calculate the wavelength in air of blue-green
  light that has a frequency of 600 1012 Hz.

11
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.

12
18.1 Waves of the electromagnetic spectrum

• Radio waves are on the low-frequency 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.

13
(No Transcript)
14
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.

15
(No Transcript)
16
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
17
18.2 Interference, Diffraction, and Polarization

• In 1807, Thomas Young (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 slits, the light fell
  on a screen.

• A pattern of alternating bright and dark bands
  formed is called an interference pattern.

18
(No Transcript)
19
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.

20
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.

21
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.

22
18.2 Grating Formula
distance between grating lines (m)
distance between 2 first order bright spots
wavelength of light (nm)
l d sinq dw L
distance between screen and glasses
d 13,500 lines/inch ? lines/m
23
18.2 Polarization

• Polarization is another wave property of light.
• The fact that light shows polarization tells us
  that light is a transverse wave.

24
(No Transcript)
25
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.

26
(No Transcript)
27
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.

28
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.

29
18.3 Special Relativity

• Key Question
• What are some of the implications of special
  relativity?

Students read Section 18.3 AFTER Investigation
18.3
30
18.3 Special Relativity

• The theory of special relativity describes what
  happens to matter, energy, time, and space at
  speeds close to the speed of light.

31
18.3 Special Relativity

• These effects are observed in physics labs
• Time moves more slowly for an object in motion
  than it does for objects that are not in motion.
  This is called time dilation.
• As objects move faster, their mass increases.
• The definition of the word simultaneous
  changes.
• Space itself gets smaller for an observer moving
  near the speed of light.

32
18.3 Speed of light paradox

• The theory of special relativity comes from
  thinking about light.

• A ball thrown from a moving train approaches you
  at the speed of the ball relative to the train
  plus the speed of the train relative to you.
• The speed of light appears the same to all
  observers independent of their relative motion.

33
18.3 Speed of light paradox

• If the person on the train were to shine a
  flashlight toward you, you would expect the light
  to approach you faster.
• The light should come toward you at 3 108 m/sec
  plus the speed of the train.
• But Michelson and Morley found experimentally
  that the light comes toward you at a speed of 3
  108 m/sec no matter how fast the train approaches
  you!

34
18.3 Consequences of time dilation

• In the early 1970s an experiment was performed by
  synchronizing two precise atomic clocks.
• One was put on a plane and flown around the
  world, the other was left on the ground.
• When the flying clock returned home, the clocks
  were compared.
• The clock on the plane measured less time than
  the clock on the ground. The difference agreed
  precisely with special relativity.

35
18.3 Einstein's formula

• This equation tells us that matter and energy are
  really two forms of the same thing.

E mc2
speed of light 3.0 x108 m/sec
Energy (J)
Mass (kg)
36
18.3 The equivalence of energy and mass

• If a particle of matter is as rest, it has a
  total amount of energy equal to its rest energy.
• If work is done to a particle by applying force,
  the energy of the particle increases.
• At speeds that are far from the speed of light,
  all the work done increases the kinetic energy of
  the particle.
• It would take an infinite amount of work to
  accelerate a particle to the speed of light,
  because at the speed of light the mass of a
  particle also becomes infinite.

37
18.3 The equivalence of energy and mass

• Einsteins was able to deduce the equivalent of
  mass and energy by thinking about the momentum of
  two particles moving near the speed of light.
• Since the speed of light must be the same for all
  observers regardless of their relative motion and
  energy and momentum must be conserved, as the
  speed of an object gets near the speed of light,
  the increase in mass must come from energy.

38
18.3 Calculate equivalents

• A nuclear reactor converts 0.7 of the mass of
  uranium to energy.
• If the reactor used 100 kg of uranium in a year,
  how much energy is released?
• One gallon of gasoline releases 1.3 108 joules.
  
• How many gallons of gasoline does it take to
  release the same energy as the uranium?

39
18.3 Simultaneity

• The two lightning strikes are simultaneous to the
  observer at rest, but the observer moving with
  the train sees the lightning strike the front of
  the train first.

40
Application Holography