1. Information from the Cosmos
Radiation
Information from the Cosmos

2. Radiation,Waves, & Information
Most of the information around us gets to us in waves.
Sound energy that travels to our ears is in one form of wave.
Light is energy that comes to our eyes if the form of another type of wave.
Energy (information) that is transferred from place to place in the form of a wave is called RADIATION.

3. 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 1930’s, 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.

4. It is neither, but it’s like both
So, what is light?
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 it’s like both
The solution was to decide that light was neither a wave nor a particle, but something else which sometimes behaved like them.

5. 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
pattern in a field (light or radio waves).

6. 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
Frequency f = how often wave crest passes, longer wavelength means lower frequency
Velocity v= speed of wave
v = f x 

7. period = 1 / frequency OR frequency = 1 / period
Frequency and Period
Frequency: how often a vibration (cycle, repetition) 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

8. Wave Speed
The speed of some waves depends on the medium through which the wave travels.
Sound waves travel at speeds of 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

9. Motion of Waves Is there a relationship between
the motion of the wave through space
and
the motion of the medium that a wave moves in?

10. Wave Types
Two types of waves
transverse
longitudinal
Cheerleader demo

11. Types of waves
Transverse waves: the motion of the medium 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, some of the waves produced in earthquakes.

12. Do waves travel through empty space?
What if there is no medium to move in?
Can any waves travel through empty space?
If so, which ones?

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

14. Terminology Radiation: Light: Electromagnetic (EM) radiation:
a way to transfer of energy in the form of a wave
Light:
another name for electromagnetic radiation
Electromagnetic (EM) radiation:
Also known as light, transfers energy and information from one place to another (in form of coupled electric and magnetic waves)
Visible light:
the range of electromagnetic radiation that the human eyes perceive as visible
Need to understand these

16. Group Question
Determine the wavelength of your group’s favorite radio station.
Assume you are 100 km (~60 miles) from the station transmitter. Calculate how long it takes for the radio waves to arrive at your location from the radio station transmitter.
Wave speed = frequency x wavelength
Speed of light (radio waves) = c = 3x 108m/sec
Distance = speed x time
x103 Hz (AM radio frequencies)
x106 Hz (FM radio frequencies)

17. 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

18. Charged Particle Interactions
Any electrically charged object exerts a force on other charged objects.
Electrons
negatively charged
Protons
positively charged
Like charges repel one another.
Unlike charges attract.

19. Electrical Force Electrical force:
is a universal force
(every charged particle affects every other charged particle)
may be attractive or repulsive force
is always directed along the line connecting two charges
depends on the product of the two charges
depends on the distance between the two charges squared
(obeys the “inverse square rule”)
Today, physicists describe electric forces in terms of an electrical field produced by the presence of electrical charge.

20. Charged Particles and Electric Fields
Electric field strength proportional to 1/r2 .
An electric field extends outward in all directions from any positively charged particle.
If a charged particle moves, its electric field changes.
The resulting disturbance travels through space as a wave.

21. Magnetic Fields
If an electric field changes with time (let’s say the source charge wiggles),
then a magnetic field is created,
coupled to the time-variant electric field.
Magnetic fields influence behavior of magnetized objects.
Earth’s magnetic field causes compass needles to point N
bar magnets
electromagnets
A simplistic explanation of magnetic fields

22. Electromagnetism
Electric and magnetic fields do not exist as independent entities.
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.

23. where h = Planck’s constant
Frequency and Energy
Light waves carry energy (E) across space.
The energy is related to the frequency of the light wave by
E = hf
where h = Planck’s constant
Recall that wave speed relates frequency and wavelength: v = f
and for light, c = f
so, E  f or E  1/

24. Creating and Detecting Light
Light is created by the motion of charged particles.
Matter is made up of atoms, which are in turn made up of charged particles.
Motions of these charged particles create light.
Not just the light we detect with our eyes, but at all wavelengths (or frequencies).

25. Electromagnetic Spectrum

26. Properties of Light Polarization Reflection Refraction Dispersion
Diffraction
Interference

27. Properties of Light: Reflection 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

28. 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.

29. Visible Light Prism will separate light into its components
Composed of 7 hues (Roy G. Biv), known as its spectrum
Red (~ 700 nm or 7000 Å)
Orange
Yellow
Green
Blue
Indigo
Violet (~ 400 nm or 4000 Å)
Color determined by its frequency (or, equivalently, its wavelength)
The sequence of colors red, orange, yellow, green, blue, and violet may be remembered by memorizing the name of that fine fellow "ROY G. BV". This was originally "ROY G. BIV", because it used to be common to call the region between blue and violet "indigo". In modern usage, indigo is not usually distinguished as a separate color in the visible spectrum; thus Roy no longer has any vowels in his last name. (from )

30. Visible Spectrum
Red Orange Yellow Green Blue Violet

31. Properties of Light: Diffraction
Diffraction is the bending of a wave as it passes through a hole or around an obstacle.
If light consists of parallel rays, they would travel through a small pinhole and make a small, bright spot on a nearby screen.
Fuzzy shadow
Sharp-edged shadow
Effect cannot be explained by ray model of light.

32. Diffraction of Waves
Actually observe a spot larger than the pinhole and varying in brightness.
The pinhole somehow affects the light that passes through it.
Diffraction is proportional to the ratio of wavelength to width of gap.
The longer the wavelength and/or the smaller the gap, the greater the angle through which the wave is diffracted.
Fuzzy shadow

33. 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.

34. 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.

35. 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.

36. 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 the average thermal energy of particles that make up that object.

37. TWO MAJOR SCALES °F and °C
Fahrenheit scale based on temperature that salt water freezes 0°F (lower than pure water).
Related to Celsius (or Centigrade) by the formula:
F = 9/5 C + 32
C = 5/9(F - 32).
Celsius is the metric temperature scale, and thus the one used by scientists

38. ABSOLUTE SCALE K AND °C
Celsius (originally Centigrade) based on freezing and boiling point of pure water, chosen to be 0°C and 100°C
Kelvin based on absolute coldest temperature possible (absolute zero)
Related by
K = C –
C = K
Kelvin is the SI unit, and thus also used by the scientific community. For a good web page on this, go to:

39. All molecular motion stops
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

40. Radiation Laws Blackbody Radiation Wien’s Law Stefan-Boltzmann Law
Planck Spectrum
Characteristics of Radiator
Wien’s Law
Relates wavelength at which a blackbody emits its maximum energy, max, to the temperature, T, of the blackbody.
Stefan-Boltzmann Law
Relates total energy emitted per second per square meter by a blackbody, E, to the 4th power of its absolute temperature T.

41. Blackbody Radiation
Consider an idealized object that absorbs all the electromagnetic radiation that falls on it - called a “blackbody.”
A blackbody absorbs all energy incident on it and heats up until it is emitting energy at the same rate that it absorbs energy.
The equilibrium temperature reached is a function of the total energy striking the blackbody each second.

42. Characteristics of Blackbody Radiation
A blackbody with a temperature higher than absolute zero emits some energy at all frequencies (or wavelengths).
A blackbody at higher temperature emits more energy at all frequencies (or wavelengths) than does a cooler one.
The higher the temperature of a blackbody, the higher the frequency (the shorter the wavelength) at which the maximum energy is emitted.

43. Blackbody Radiation
Blackbody radiation: the distribution of radiation emitted by any heated object.
The curve peaks at a single, well-defined frequency and falls off to lesser values above and below that frequency.
The overall shape (intensity vs frequency) is characteristic of the radiation emitted by any object, regardless of its size, shape, composition, or temperature.

44. Planck Spectrum
As an object is heated, the radiation it emits peaks at higher and higher frequencies.
Shown here are curves corresponding to temperatures of K (room temperature), 1000 K (glow dull red), K (red hot), and K (white hot).

45. “Red Hot”
As something begins to heat-up, there probably isn’t any visible information to tell you it is warming up.
Once it starts to glow red, you have learned it’s hot – don’t touch.
Like the stove burners.
As it continues getting hotter, it changes to orange, then yellow, green, blue and white.
White because all visible wavelengths are represented fairly evenly. With light, black is the absence of color (wavelengths), and white is the presence of all color (wavelengths). This is opposite of pigments where white is absences of color and black is all color combined. Rarely will something have a “sharp blackbody peak” in green, which is why it is rare to see.

46. This relationship is known as Wien’s law.
The Sun and stars emit energy that approximates the energy from a blackbody.
It is possible to estimate their temperatures by measuring the energy they emit as a function of wavelength - that is, by measuring their color.
The wavelength at which a blackbody emits its maximum energy can be calculated by
 max = 3,000,000 / T
where the wavelength  max is in nanometers (10-9 m)
and the temperature T is in kelvin.
This relationship is known as Wien’s law.

47. Hotter objects are brighter and “bluer” than cooler objects.
Effect of Temperature
Hotter objects are brighter and “bluer” than cooler objects.

48. Getting Warmer Top picture: a cool gas cloud.
2nd from top: a “cool” infrared emitting star appears reddish. Notice how little the other colors contribute relative to red.
2nd from bottom: our star, the Sun
Bottom picture: a star cluster of very hot UV stars. Notice how they appear white.

49. Electromagnetic Radiation

50. Problem - Wien’s law
The average surface temperature of the Sun is about 5800 K. At what wavelength is maximum energy emitted from the Sun?
If T = 5800 K
and max = 3,000,000 / T ,
then max = 3,000,000 / 5800 = 520 nm.
520 nm is at the middle of the visible light portion of the electromagnetic spectrum.
The human eye is most sensitive to the wavelengths at which the Sun puts out the most energy.

51. Stefan-Boltzmann Law
If add up the contributions from all parts of the E-M spectrum, obtain the total energy emitted by a blackbody over all wavelengths.
That total energy emitted per second per square meter by a blackbody at temperature T is proportional to the 4th power of its absolute temperature.
This is known as the Stefan-Boltzmann law,
E = T where E stands for the total energy and  is a constant number.

52. Problem - Stefan-Boltzmann Law ET = T4
The average surface temperature of the Sun is about 5800 K If the Sun were twice as hot, T = 2 x 5800 K = 11,600 K, how much more energy would it radiate than it does now?
The energy radiated by the Sun would be 24 or 16 times more than now.

53. Electromagnetic Spectrum

54. Electromagnetic Energy from the Sun

55. Why Do We Need Space Telescopes?
Because not all light can make it through the atmosphere – which is a good thing in some cases. Such as Gamma ray, x-ray, and most UV radiation are unable to penetrate the atmosphere and reach down to the ground. If the atmosphere is said to be transparent to a wavelength of light, then that wavelength actually reaches the ground, such as visible light (which is why we see it). Thus, the atmosphere is said to have a window to that wavelength. If the atmosphere is said to be opaque to a wavelength, then it is unable to penetrate the atmosphere and reach the ground. As you can see from the diagram above and on page 71 of your text, some forms of radiation can penetrate to a certain depth, but never actually reach the ground where we are, like infrared.

56. 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.
Wavelength (angstroms)
Half-Absorption Altitude (km)