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

http://www.nyit.edu/

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

http://en.wikipedia.org/wiki/File:Photoelectric_effect.svg

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

http://upload.wikimedia.org/wikipedia/commons/2/26/Diffraction_pattern_in_spiderweb.JPG

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




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:

http://upload.wikimedia.org/wikipedia/commons/8/82/Pahoehoe_toe.jpg

http://upload.wikimedia.org/wikipedia/en/0/00/Blackbody-colours-vertical.svg

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




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


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

Visible Objects in the Universe

http://www.nyit.edu/

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


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...


• 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


• 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/

http://upload.wikimedia.org/wikipedia/en/c/c3/Solar_sys8.jpg


• 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.


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


speeding toward us at 500,000 km/sec!will arrive in 4 billion years!

Andromeda


Elliptical galaxy Irregular galaxy

we are here

Group Activity

our cosmic address


Describe what you see. What are some of the interesting features?


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.


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?


Assuming there are 100 billion galaxies in the visible universe, what fraction of the sky is covered by the HUDF image?


How many planets are there in the visible Universe?


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?


13.7 billion years in one image


• Planets

• Stars

• Galaxies


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




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

Light (Electromagnetic Radiation) & Its Nature

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