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 Huygens It’s a Particle It’s a Wave
Light has a Wave Like Nature Longitudal vs. Transverse wave: http://paws.kettering.edu/~drussell/Demos/waves/wavemotion.html
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 • 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 Photoelectric effect 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)
The Electromagnetic Spectrum colors seen in a spider web are partially due to dispersion Isaac Newton
The Electromagnetic Spectrum Higher energy Lower Energy
-15 10 m gamma rays X-rays Ultra-violet Visible Infrared Microwave Radio waves Many wavelengths of light outside of visible 1000 km
} } } } } } .... 4-5 octaves above visible - X-rays ....several more octaves below - microwave radiation ... octave of “visible” light ... one octave below “visible” - infrared radiation ... entire spectrum ... entire spectrum Sound you’re hearing represents.... adopted from Prof. David Helfand at Columbia University
-15 10 m 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 1000 km
All information in Astronomy comes from collecting light using instruments called telescopes Location of Telescope Installations ?
Location of Telescope Installations ? different wavelengths... different considerations The 100 inch (2.5 m) Hooker telescope at Mount Wilson Observatory near Los Angeles, California.
Making use of EM radiation Reflected and Emitted Light The Andromeda Galaxy at different Wavelengths:
Thermal “blackbody” radiation • 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
The temperature of an object determines what type of EM (light) it will emit. 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) Comparison of the fahrenheit, celsius, and kelvin scales Credit: NASA
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
Thermal “blackbody” radiation • 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 Hotter object (shorter λ) brighter Cooler object (longer λ) dimmer every object emits radiation that depends on its temperature: Cooler objects are redder than hotter objects Cooler objects are dimmer than hotter objects
Thermal Black Body Radiation: 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
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?
Thermal “blackbody” radiation • 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 Wein’s Law: every object emits radiation that depends on its temperature: Cooler objects are redder than hotter objects Cooler objects are dimmer than hotter objects
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
Hubble Space Telescope 2.4m optical telescope resides in orbit of Earth
The Hubble Ultra Deep Field What objects do we see here?
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. • Planets: an object that orbits a star, is large enough to have settled into a round shape and dominates its orbital zone;
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.
Dynamical effect presents a feature of clear distinction between planets and other bodies 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 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)
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. • 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;
stars are point sources cross-like spikes in image (diffraction spikes) caused by strong + concentrated light A sample of stars 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.