Chapter 3: Radiation

Information from the Cosmos

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.

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 (Newton’s 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.

So, what is light? continued

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

• 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?

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

Is it a wave?

Is it a particle?

It is neither,but it’s

like both

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

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.

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).

Waves: Standard DimensionsIn physics, waves are described by a few standard

dimensions.

Frequency f = how often wave crest passes,longer wavelength means lower frequency

Frequency f = how often wave crest passes,longer wavelength means lower frequency

v = f x v = f x

Wavelength = length of one cycleWavelength = length of one cycleAmplitude A= height of wave above “rest position”Amplitude A= height of wave above “rest position”

Velocity v= speed of wave Velocity v= speed of wave

Frequency and PeriodFrequency: 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

Questions: Frequency and Period

1. What is the frequency in cycles per second of a 50-hertz wave?

Frequency = 1 / period period = 1 / frequency

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.

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

Wave Types

• Two types of waves

–transverse

–longitudinal

Cheerleader demo

Types of wavesTransverse waves: the motion of the medium (rope) is at right

angles to the direction in which the wave travels.

Longitudinal waves: the particles in the medium move along the direction of the wave;

travel in solids, liquids, and gases.

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.

Examples: sound waves, one type of Slinky wave shown in class, some of the waves produced in earthquakes.

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

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

Charged Particle InteractionsAny electrically charged object exerts a force on

other charged objects.

Like charges repel one another.

Unlike charges attract.

Protons positively charged

Electrons negatively charged

Charged Particles and Electric Fields

An electric field extends outward in all directions

from any charged particle.

If a charged particle moves, its electric field changes.

The resulting disturbance travels through space as a

wave.

Electric field strength proportional to 1/r2 .

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.

Electromagnetic radiation• Light is just one type out of

many types of “electromagnetic radiation” (EMR).

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

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

Electrons accelerate and

deceleratereleasing

energy in the form of EMR

Electrons drop to lower energy

levelsreleasing

energy in the form of EMR

Observing EMR• When EMR is absorbed or detected (e.g. by a

leaf, an eye, a telescope or photographic film) the reverse happens.

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

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

EMR is absorbed by electrons

and is turned into an electrical

signal

EMR is absorbed by

electronsallowing a

reaction to take place

Electromagnetic Spectrum

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

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.

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”.

• wavelength

• frequency• 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.

• 450 nm • 700 nm

• ultraviolet • infra-red

• 6 x 1014 Hz

Visible Spectrum

Red Orange Yellow Green Blue Violet

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.

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.

Electromagnetic Spectrum

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.

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.

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

Electromagnetic Radiation

Type ofRadiation

WavelengthRange (nm)

Radiated byObjects at thisTemperature

Typical Sources

Gamma rays Less than0.01

More than108 K

No astronomical sources thishot; some produced in nuclearreactions.

X rays 0.01 – 20 106 – 107 K Gas in clusters of galaxies;supernova remnants; solarcorona.

Ultraviolet 20-400 105 – 106 K Supernova remnants; veryhot stars.

Visible 400-700 103 – 105 K Stars

Infrared 103 – 106 10 – 103 K Cool clouds of dust and gas,planets, satellites

Radio More than106

Less than 1 K No astronomical objects thiscold: radio emissionproduced by electronsmoving in magnetic fields

Electromagnetic Energy from the Sun

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)

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The Doppler Effect and Relative Motion

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.

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

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 “c”and 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?