Light (Electromagnetic Radiation) & Its Nature
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Light (Electromagnetic Radiation) & Its Nature
Light (Electromagnetic Radiation) & Its Nature
Light:
also referred to as electromagnetic radiation (EM radiation)
form of energy that transverses through space
is the source of information about the Universe
Newton vs HuygensIt’s a Particle
It’s a Wave
Longitudal vs. Transverse wave:
http://paws.kettering.edu/~drussell/Demos/waves/wavemotion.html
Light has a Wave Like Nature
Key Parameters of Light as a Wave
A - Amplitude – Vertical height of maxima or depth of minima of a wave proportional to intensity /brightness of light λ - Wavelength – Distance between two adjacent maxima f - Frequency – Number of maxima that pass a certain point in a second Remember, light is a form of energy (E)
λ f Eλ f E
Light travels as a transverse wave
Transverse wave – direction of vibration is perpendicular to its direction of travel. Longitudinal Wave – direction of vibration is the same as its direction of travel
No Light
Light has a Particle-Like Nature
Photoelectric effect• Ejection of electrons from metal
surfaces by photon impact
• Minimum photon energy (frequency) needed to overcome electron binding PE
• Additional photon energy goes into KE of ejected electron
• Intensity of light related to number of photons, not energy
• Application: photovoltaic cells
Light is a stream of particles, called photonsE=hf
So Which is It?Certain properties of light are best described
by thinking of it as a wave, while others are best described by thinking of it as a stream
of particles. Both waves and particles transit energy
through space from one part of the universe to the next
Light interacts with matterInteraction begins at surface and
depends onSmoothness of surfaceNature of the material
Angle of incidence
Possible interactionsReflectionRefraction Absorption
Transmission
Transparent materials transmit light
Opaque materials do not allow transmission of light
(reflect, absorb or combination)
The Electromagnetic Spectrum
Isaac Newton
colors seen in a spider
web are partially due to
dispersion
The Electromagnetic Spectrum
Higher energy Lower Energy
gamma rays
X-rays
Ultra-violetVisible
Infrared
Microwave
Radio waves
Many wavelengths of light outside of visible
10 m-15
1000 km
}... one octave below “visible” - infrared radiation
... entire spectrum}} ... octave of “visible” light
Sound you’re hearing represents....
} ....several more octaves below - microwave radiation
} .... 4-5 octaves above visible - X-rays
... entire spectrum}adopted from Prof. David Helfand at Columbia University
gamma rays
X-rays
Ultra-violetVisible
Infrared
Microwave
Radio waves
• Many wavelengths of light outside of visible
• Astronomers must consider the full EM spectrum
10 m-15
1000 km
Location of Telescope Installations ?
All information in Astronomy comes from collecting light using instruments called telescopes
Location of Telescope Installations ?
The 100 inch (2.5 m) Hooker telescope at Mount Wilson Observatory near Los Angeles, California.
different wavelengths...different considerations
Making use of EM radiation
Reflected and Emitted Light
The Andromeda Galaxy at different Wavelengths:
Images from the Spitzer and Chandra space telescope web sites
• The energy emitted per second by an object at different wavelengths is called its spectrum
• An object emits a thermal radiation spectrum due to its temperature
Thermal “blackbody” radiation
Comparison of the fahrenheit, celsius, and kelvin scales Credit: NASA
Temperature: the quantity that tells how warm or cold an object is with respect to some standard. It is a measure of the average kinetic energy of the
molecules or atoms in an object. scales: Celsius (°C), Fahrenheit (°F), or Kelvin (K)
The temperature of an object determines what type of EM (light) it will emit.
Converting between F, C, K
T(°F) = 9/5 T(°C)+32°T(°C) = 5/9 (T(°F)-32°)T(°K)=T(°C)+273.15
every object emits radiation that depends on its temperature: Cooler objects are redder than hotter objects Cooler objects are dimmer than hotter objects
• The energy emitted per second by an object at different wavelengths is called its spectrum
• An object emits a thermal radiation spectrum due to its temperature
Thermal “blackbody” radiation
Hotter object (shorter λ) brighter
Cooler object (longer λ) dimmer
As temperature increases, the glow color changes from red to yellow to white to blue.
The temperature of a lava can be estimated by observing its color: lava flows at about 1,000 to 1,200 °C.
Thermal Black Body Radiation:
Black-body laws can be applied to human beings. For example, some of a person's energy is radiated away in the form of electromagnetic radiation, most of which is infrared
Thermal Black Body Radiation:
Infrared PictureWhat are we looking at?
Why does it appear this way?
Same picture, no humans.
Why is the spot in the middle brighter?
every object emits radiation that depends on its temperature: Cooler objects are redder than hotter objects Cooler objects are dimmer than hotter objects
• The energy emitted per second by an object at different wavelengths is called its spectrum
• An object emits a thermal radiation spectrum due to its temperature
Thermal “blackbody” radiation
Wein’s Law:
Light transverses electromagnetic energy
through space at c = 3.0 108m/s
How long does it take light to travel one meter?
3.3 ns of “look-back” time
On the Moon
Time-delay
Light (Electromagnetic Radiation) & Its Nature
Key Concepts for Week-3, Class-1:
(what You need to know, as You will be tested on this material):
Dual nature of light: wave-like nature (double-slit experiment) & particle-like nature (photoelectric effect experiment)
Connection between wavelength, frequency and energy Distinction between transverse & longitudinal wave
Phenomena: reflection, refraction, absorption, and transmission The span of EM radiation: radio-waves, microwaves, infrared
light, visible light, ultra-violet light, X-rays, gamma rays Thermal “blackbody-radiation” spectrum
Temperature and its units (Fahrenheit, Celsius, Kelvin) Concept of look-back time
Light year (ly) as a measure of distance
The Hubble Ultra Deep Field
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.
Hubble Space Telescope2.4m optical telescoperesides in orbit of Earth
The Hubble Ultra Deep Field
What objects do we see here?
• Planets: an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone;
Objects in a Visible UniverseThe Universe is defined as the summation of all particles and energy that exist and the space-time in/during which all events occur.
What is a Planet?
Conventional (past) definition: Planet is a body that orbits a star, shines by reflecting the star’s light and is larger than an asteroid.
What observation ignited the debate about the definition of a planet?
Observation of the vast population of objects in the vicinity of Pluto (Kuiper Belt Objects = KBO);
In particular, KBO Eris is larger than Pluto; If Pluto is a planet, not only Eris but also dozen of other
KBO objects will need to be considered a planet.
Key Feature: Planet is a body massive enough to dominate its orbital zone by a) flinging smaller bodies away , b) sweeping them up in direct collisions, or c) holding them in stable orbits
Dynamical effect presents a feature of clear distinction between planets and other bodies
Another way of stating the definition: a body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit.
Proxy is µ= M(planet)/M(objects)
Earth is a pretty big rocky planet....
but not very big as planets go...
• Planets: an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone;
• Stars: massive gaseous body in outer space, just like the Sun.Unlike a planet, a star generates energy through nuclear fusionand emits visible light;
Objects in a Visible UniverseThe Universe is defined as the summation of all particles and energy that exist and the space-time in/during which all events occur.
Stars are in a dynamic balance between gravity and pressure
A sample of starsstars are point sourcescross-like spikes in image (diffraction spikes)caused by strong + concentrated light
stars~ 109m
Super Nova: explosion of the star
One of the most energetic explosive events known is a supernova. These occur at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy.
and tiny compared to ordinary stars,
even smaller when compared to giant stars,
and invisible compared to supergiants
planets~ 107m
Planet sizes are to scale, but distance is not
http://www.youtube.com/watch?v=HEheh1BH34Q
• Planets: an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone;
• Stars: massive gaseous body in outer space, just like the Sun.Unlike a planet, a star generates energy through nuclear fusion and therefore emits light;
Objects in a Visible Universe
The Universe is defined as the summation of all particles and energy that exist and the space-time in/during which all events occur.
http://janus.astro.umd.edu/SolarSystems/
• Planets: an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone;
• Stars: massive gaseous body in outer space, just like the Sun.Unlike a planet, a star generates energy through nuclear fusion and therefore emits light;
• Galaxies: a large aggregate of stars (as well as othermaterials such as gas, dust, and dark matter), held in association by their mutual gravity, and relatively isolated from other such aggregates. Usually grouped into three main types: Spiral, Elliptical, and Irregular.
Objects in a Visible Universe
The Universe is defined as the summation of all particles and energy that exist and the space-time in/during which all events occur.
http://janus.astro.umd.edu/SolarSystems/
http://www.seasky.org/celestial-objects/stars.html
A sample of galaxies
Spiral galaxylike our galaxy the Milky Way....
galaxies~ 1021m
A sample of galaxies
speeding toward us at 500,000 km/sec!will arrive in 4 billion years!
Andromeda
A sample of galaxies
Elliptical galaxy Irregular galaxy
we are here
Group Activity
our cosmic address
The Hubble Ultra Deep Field
Describe what you see. What are some of the interesting features?
The Hubble Ultra Deep Field
Look at the objects
Think about the time it took for “info” to arrive
Think about their colors; What can you tell about their temperature?
The Hubble Ultra Deep Field
Look at the objects
Think about the time it took for “info” to arrive
Think about their colors; What can you tell about their temperature?
Which way did the Hubble Space Telescope point when taking the Hubble Ultra Deep Field?
Estimate how many galaxies are in this image.
The Hubble Ultra Deep Field
How many galaxies are there in the visible Universe?
How can we use this image to figure out the number of galaxies in the Universe?
The Hubble Ultra Deep Field
Assuming there are 100 billion galaxies in the visible universe, what fraction of the sky is covered by the HUDF image?
The Hubble Ultra Deep Field
How many planets are there in the visible Universe?
The Hubble Ultra Deep Field
67
Is this really the only planet in the only solar system in the only galaxy that’scomfortable for life?
http://www.youtube.com/watch?v=wJXSSYyIVqw&feature=related
How do you read time in this image?
The Hubble Ultra Deep Field
13.7 billion years in one image
The Hubble Ultra Deep Field
• Planets
• Stars
• Galaxies
Objects in a Visible Universe
We are still in mostly “in the dark”…
What evidence do we have for dark matter?
What evidence do we have for dark energy?
only ~ 4% ordinary matter !
present at ~ 23 %
present at ~ 73 %
http://burro.cwru.edu/JavaLab/GalCrashWeb/main.html
STUDENTS:
NEXT WEEK PLEASE BRING LAPTOPS (1 OR 2 PER GROUP)
& PRIOR TO COMING TO CLASS, UPLOAD THE FOLLOWING WEBSITE INTO THE “CASH” MEMORY
Light (Electromagnetic Radiation) & Its Nature
Key Concepts for Week-3, Class-2:
(what You need to know, as You will be tested on this material):
Definitions: Planets
Stars Galaxies
nuclear fusion reactions within stars
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