Chapter 3: Radiation

1
Chapter 3 Radiation

• Information from the Cosmos

2
Information from the Cosmos

• Until recently, our knowledge of the universe was
  obtained only by studying the visible light that
  happened to arrive on Earth.
• Since the 1930s, possible to study other types
  of radiation and particles ---
• radio waves, X-rays, gamma rays, cosmic rays,
  neutrinos, and gravitational radiation.
• To understand the methods used to study the
  cosmos, we must understand the basic nature and
  behavior of light.

3
So, what is light? The Historical View

• Greeks
• 5th century B.C., Socrates and Plato speculated
  that light was made up of streamers emitted by
  the eye, acting like antennae (you see when
  antennae make contact with an object).
• Pythagoreas believed that light traveled from
  luminous objects to the eye in the form of tiny
  particles.
• Empedocles taught that light traveled in waves.
• Newton championed particle theory of light.
• Huygens (Newtons contemporary) stated that light
  was a wave supported the assertion with
  experimental data showing that, under certain
  circumstances, light will spread out (diffract)
  like a wave.
• Einstein published a theory (photoelectric
  effect) in 1905 that proposed light to be
  massless bundles of concentrated electromagnetic
  energy - called photons.

4
So, what is light? continued

• The particle or ray model of light is illustrated
  by the properties of reflection and refraction.

Is it a particle?

• The wave model of light is illustrated by the
  properties of reflection, refraction,
  diffraction, interference, and polarization.

Is it a wave?

• But there are problems if light is a
  wave, and waves need a medium such as air or
  water to carry them, then how can light travel
  through empty space?

It is neither,but itslike both

• The solution was to decide that light was neither
  a wave nor a particle, but something else which
  sometimes behaved like them.

5
Waves and Information

• Most of the information around us gets to us in
  waves.
• Sound energy that travels to our ears is in one
  form of a wave.

• Light is energy that comes to our eyes if the
  form of another type of wave.

6
What is a Wave?

• Wave motion is NOT a mechanical phenomenon
  because a wave is not a material object
  but a form.
• It cannot be assigned a mass, and the concept of
  acceleration cannot be applied to a wave.
• The motion of a wave is vastly different from the
  motion of the medium in which it travels. In
  fact, a wave can exist without any movement of
  matter at all!
• So, what is a wave? It is a pattern or form that
  moves.
• It can be
• a deformation of a material object
  (music string or waves on the surface of a body
  of water)
• OR
• it can be a pattern in a field ( light or radio
  waves).

7
Waves Standard Dimensions

• In physics, waves are described by a few standard
  dimensions.

Amplitude A height of wave above
rest position
Wavelength ? length of one cycle
Velocity v speed of wave
v f x ?
Frequency f how often wave crest
passes,longer wavelength means lower frequency
8
Frequency and Period
Frequency how often a vibration occurs in some
interval of
time, vibrations (or cycles) per unit time.
units are Hertz (Hz) 1-Hz 1 vibration/sec
1 cycle/sec
103 Hz kHz (AM radio frequencies)
106 Hz MHz (FM radio frequencies)
Period the time to complete one vibration (or
cycle),
the inverse of the frequency
period 1 / frequency OR frequency 1 /
period
9
Questions Frequency and Period
Frequency 1 / period
period 1 / frequency
1. What is the frequency in cycles per second of
a 50-hertz wave?
A 50-hertz wave vibrates 50 times per
second.
2. The Sears Building in Chicago sways back and
forth at a vibration frequency of about 0.1 Hz.
What is its period of vibration?
The period is 1/frequency 1/(0.1 Hz)
1/(0.1 vibrations/s) 10 s. Thus, each cycle
(or vibration) takes 10 seconds.
10
Wave Speed

• The speed of some waves depends on the medium
  through which the wave travels.
• Sound waves travel at speeds of 330 m/s to 350
  m/s in air, and about four times as fast in
  water.
• The speed of the wave is related to the frequency
  and wavelength of the wave.
• Wave speed frequency x wavelength

11
Wave Types

• Two types of waves
• transverse
• longitudinal

Cheerleader demo
12
Types of waves
Transverse waves the motion of the medium
(rope) is at right angles to
the direction in which the wave travels.
Examples stretched strings of musical
instruments, waves on the surfaces of
liquids, some of the waves produced
in earthquakes. Although they require no
medium to travel, electromagnetic waves are
also transverse waves.
Longitudinal waves the particles in the medium
move along the direction of the
wave travel in solids, liquids, and gases.
Examples sound waves, one type of Slinky wave
shown in class, some of the waves
produced in earthquakes.
13
Light as a Wave

• Light is a type of electromagnetic radiation that
  travels through space as a wave.
• Electromagnetic waves are fundamentally different
  from many other waves that travel through
  material media (sound or water waves).
• Electromagnetic waves require NO material medium
  to travel from place to place.
• The wave speed of all types of electromagnetic
  radiation in a vacuum is called the speed of
  light. c 300,000 km/sec

14
Creating Electromagnetic Waves

• All matter is made up of atoms.
• Atoms are, in turn, made up of smaller particles
  protons, electrons, and neutrons.
• Two of the elementary particles that make up
  atoms possess a property described as electrical
  charge.
• The charges on each are equal and opposite.
• electron - charge
• proton charge

15
Charged Particle Interactions

• Any electrically charged object exerts a force on
  other charged objects.

Protons positively charged
Electrons negatively charged
Like charges repel one another.
Unlike charges attract.
16
Charged Particles and Electric Fields

• An electric field extends outward in all
  directions from any charged particle.

Electric field strength proportional to 1/r2 .
If a charged particle moves, its electric field
changes. The resulting disturbance travels
through space as a wave.
17
Electromagnetism

• A changing electric field produces a magnetic
  field.
• The vibrating electric and magnetic fields are
  always oriented perpendicular to one another and
  move together through space.
• These fields do not exist as independent
  entities rather, they are different aspects of
  a single phenomenon electromagnetism(EMR).

Together, they constitute an electromagnetic wave
that carries energy and information from one part
of the universe to another.
18
Electromagnetic radiation

• Light is just one type out of many types of
  electromagnetic radiation (EMR).

Electrons accelerate and decelerate
Electrons drop to lower energy levels

• EMR is produced when electrons decelerate and
  lose energy (e.g. in a radio transmitter)

releasing energy in the form of EMR
releasing energy in the form of EMR

• or drop from a high energy level in an atom to a
  lower one and lose energy.

19
Observing EMR

• When EMR is absorbed or detected (e.g. by a leaf,
  an eye, a telescope or photographic film) the
  reverse happens.

EMR is absorbed by electrons
EMR is absorbed by electrons
allowing a reaction to take place

• The energy of the EMR is absorbed by electrons
  and converted to electrical energy

and is turned into an electrical signal

• or it causes an electron to jump to a higher
  energy level, allowing a chemical reaction to
  take place

20
Electromagnetic Spectrum
21
Properties of LightReflection and Refraction

• An isolated light beam travels in a straight
  line.
• Light can change directions under certain
  conditions
• Reflection from a surface,
• mirrors, objects
• Refraction (or bending of a ray of light) as
  the ray travels from one transparent medium to
  another.
• pencil in a clear glass of water
• light through a piece of glass

22
Properties of Light Dispersion

• Electromagnetic waves interact with the charged
  particles in matter and travel more slowly in
  transparent media than in a vacuum.
• The change in speed of the light wave causes the
  wave to refract.
• Since the speed of an EM wave in a medium changes
  with wavelength, the amount of refraction depends
  on the wavelength.
• This effect is called dispersion.

23
Color of electromagnetic radiation

• The human eye interprets difference in frequency
  as colour, and calls the range of frequencies
  that we can see visible light.

• 700 nm

• 450 nm

• wavelength

• ultraviolet

• infra-red

• frequency

• 6 x 1014 Hz

• There are of course an infinite number of
  possible frequencies (and wavelengths) for light,
  but humans can see only a very small band of
  them between the ultraviolet and the infra-red.

24
Visible Spectrum
25
Properties of Light Interference and
Superposition

• What happens if two waves run into each other?
• Waves can interact and combine with each other,
  resulting in a composite form.
• Interference is the interaction of the two waves.
• reinforcing interaction constructive
  interference
• canceling interaction destructive
  interference
• Superposition is the method used to model the
  composite form of the resulting wave.

26
Interference of Waves

• Interference ability of two or more waves
  to reinforce or cancel each other.

Constructive interference occurs when two wave
motions reinforce each other, resulting in a wave
of greater amplitude.
Destructive interference occurs when two waves
exactly cancel, so that no net motion remains.
27
Electromagnetic Spectrum
28
Radiation and Temperature

• What determines the type of electromagnetic
  radiation emitted by the Sun, stars, and other
  astronomical objects? Temperature
• Electromagnetic radiation is emitted when
  electric charges accelerate, changing either the
  speed or the direction of their motion.
• The hotter the object, the faster the atoms move
  in the object, jostling one another, colliding
  with more electrons, changing their motions with
  each collision.
• Each collision results in the emission of
  electromagnetic radiation- radio, infrared,
  visible, ultraviolet, x-rays. How much of each
  depends on the temperature of the object
  producing the radiation.

29
Measuring Temperature

• Atoms and molecules that make up matter are in
  constant random motion.
• Temperature is a direct measure of this internal
  motion.
• The higher the temperature, the faster (on
  average) the random motion of particles in
  matter.
• Temperature of an object represents average
  thermal energy of particles that make up that
  object.

30
Temperature Scales
Temperature Scale Hydrogen fuses Water boils Water freezes All molecular motion stops
Fahrenheit 18,000,032oF 212oF 32oF -459oF
Celsius 10,000,000oC 100oC 0oC -273oC
Kelvin 10,000,273 K 273 K 373 K 0 K
31
Electromagnetic Radiation
32
Electromagnetic Energy from the Sun
33
Opacity of the Atmosphere

• Only a small fraction of the radiation produced
  by astronomical objects actually reaches our eyes
  because atoms and molecules in the Earth's
  atmosphere absorb certain wavelengths and
  transmit others.
• Opacity is proportional to the amount of
  radiation that is absorbed by the atmosphere.

34
The Doppler Effect and Relative Motion
35
Doppler Effect

• Observers in a fast-moving spacecraft will see
  the stars ahead of them seem bluer than normal,
  while those behind are reddened.
• The stars have not changed their properties
  the color changes are the result of the
  motion of the spacecraft relative to the stars.

36
Red-shift and Blue-shift

• Wave motion from a source toward an observer at
  rest with respect to the source.
• Waves from a moving source "pile up" in the
  direction of motion and be "stretched out" on the
  other side.
• An observer situated in front of the source
  measures a shorter-than-normal wavelength a
  blueshift while an observer behind the source
  sees a redshift.

Apparent wavelength true frequency 1
recession velocity true wavelength
apparent frequency wave speed
37
Review - Chapter 3

• Define the following wave properties
  period, wavelength, amplitude, and frequency.
• State relationship between wavelength, frequency
  and wave speed.
• Define/describe the following terms as they
  relate to waves diffraction, superposition,
  interference, and dispersion.
• What is cand why is it special?
• What colors combine to make white light?
• What do radio waves, infrared radiation, visible
  light, ultraviolet radiation, X rays, and gamma
  rays have in common? How are they different?
• What is opacity? How does it affect observations
  on Earth?
• What is the Doppler effect? How does it alter
  observations?
• What is temperature?