# Light (Electromagnetic Radiation) & Its Nature. Light:  also referred to as electromagnetic radiation (EM radiation)  form of energy that transverses.

## Presentation on theme: "Light (Electromagnetic Radiation) & Its Nature. Light:  also referred to as electromagnetic radiation (EM radiation)  form of energy that transverses."— Presentation transcript:

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

It’s a Particle It’s a Wave

Longitudal vs. Transverse wave: http://paws.kettering.edu/~drussell/Dem os/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  E  λ  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 photons E=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 matter Interaction begins at surface and depends on Smoothness of surface Nature of the material Angle of incidence Possible interactions Reflection Refraction Absorption Transmission Transparent materials transmit light Opaque materials do not allow transmission of light (reflect, absorb or combination)

Reflection of light explained by both the particle and wave theories Refraction of light explained by both the particle and wave theories

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-violet Visible 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-violet Visible 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 Picture What 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 10 8 m/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

Visible Objects in the Universe

The Hubble Ultra Deep Field

Hubble Space Telescope 2.4m optical telescope resides in orbit of Earth

The Hubble Ultra Deep Field What objects do we see here?

an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone 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 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.

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...

an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone 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 emits visible 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.

Stars are in a dynamic balance between gravity and pressure

A sample of stars stars are point sources cross-like spikes in image (diffraction spikes) caused by strong + concentrated light stars ~ 10 9 m

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 ~ 10 7 m Planet sizes are to scale, but distance is not http://www.youtube.com/watch?v=HEheh1BH34Q

an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone 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/

an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone 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 other materials 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 galaxy like our galaxy the Milky Way.... galaxies ~ 10 21 m

A sample of galaxies speeding toward us at 500,000 km/sec! will arrive in 4 billion years! Andromeda

A sample of galaxies Elliptical galaxyIrregular 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’s comfortable 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.htmlSTUDENTS: 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