Read BEFORE coming to class: Lecture 7 Phys 1810 Read BEFORE coming to class: Electromagnetic Radiation 3.1 to 3.4 Energy Thermal Radiation Box 3-2 Flux and Luminosity (L equation in Box 17-2) Spectra 4.1, 4.2 Kirkhhoff’s Laws Radio Emission 18.4 Doppler shift: 3.5, Box 3-3, 4.5 Password change on Today! Given out in Class, not email.

Radiation Cygnus region of the Milky Way Galaxy. Supernova Remnants, star forming clouds, gas and dust. None of the data used to create this image are seen with the human eye (i.e. invisible!) Jayanne English and A. Russ Taylor for CGPS

This is important because everything we know about the universe is discovered by observing radiation from space.

The Dual Nature of Light summary Text Recall column Text Two Theories Historically Waves (Huygens) Particles (Newton) both needed in modern times Wave Theory Phenomena diffraction Interference Particle Theory photons spectral lines black bodies Macroscopic scale. One can think of photons as packets of waves. Atomic scale.

Demo

ν == nu # crests/sec  1/s == Hertz = Hz.

λ == lambda v = distance/time and lambda is the distance and Period is the time. For light v ==c and the P=1/frequency --- substitute these into the velocity equation.

Your eye sees at 550nm. What is the frequency of this light?

[EM] Time to snap fingers 1/10 sec => light travels ¾ of distance around the Earth.

Frequency & Wavelength summary Text Recall column Text The speeds of both waves are the same The upper wave has a shorter wavelength and a higher frequency The lower wave has a longer wavelength and a lower frequency High frequency: more waves pass per second! This wavelength is twice as long

Light is small part of EM spectrum Light is small part of EM spectrum. Prism bends light: “violet bends violently”. Recall 10**9 nm/m and 10**10 angstroms/m. (angstrom == A with circle on top.) Eyes mostly sensitive to 550nm (5500 A).

Created by vibrating charges. Waves in what? Electrical Force: attractive or repulsive. Every charged particle has electic field. EM waves need no medium. Created by vibrating charges. Radiation travels through a vacuum – no medium - so waves in what? [Efld == electric field; F == force; ptl == particle] Electrical force: e.g. static cling, sparks off cats. If another charged ptl in fld, then it will be attracted or repulsed. Efld Force inversely proportional to distance^2, like gravity. Cause a charge to vibrate (move up & down) , say due to heat or collisions,  wave travels through the Efld. i.e. Efld changes due to a disturbance which travels through space as a wave. F of Efld on distant ptl changes.

 a B field accompanies a changing E field. Magnetic Field (B field) analogous to E field. for magnetized objects B fields exert a force on moving charged particles. moving charged particles create B fields.  a B field accompanies a changing E field. Demos to show link between E & B fields. Objects like magnetized needle in compass, Earth’s magnetic poles. B is conventionally written with an arrow on top.

Note that the waves are perpendicular.

Electromagnetic Wave summary Text Recall column Text See another good animation: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35 oscillations occurring perpendicular to the direction of energy transfer oscillating electric & magnetic fields

 a B field accompanies a changing E field. Thus a vibrating charged particle in a star create EM waves in its own EM field and these waves propagate through space. Objects like magnetized needle in compass, Earth’s magnetic poles. B is conventionally written with an arrow on top.

Hole in wall

Hole in the Wall Expected for particles Observed

What happens if you send photons one at a time through a double slit? Check the class website for videos! Double Slit  Interference Pattern http://www.olympusmicro.com/primer/java/doubleslitwavefronts/index.html What happens if you send photons one at a time through a double slit? You can do a version of the double slit at home using a laser pointer and covering a small comb with tape, except for 2 slits. Some light sources emit 1 photon at a time, so this experiment has been done. Light is both a wave and a particle ! Would you get only 2 strips as if the photons were “baseballs” ? https://www.youtube.com/watch?v=MbLzh1Y9POQ Demonstrates the DUAL NATURE of light.

Particle Description  Photons

Photon Energy (E) h== Planck constant. but So Higher frequencies have higher energies Photon energy. h is a constant that would be supplied on a test or exam. How does the speed of radio waves compare to the speed of visible light? They both travel at the same speed.

Note: Visible light isn’t special Relation between wavelength & frequency Atmospheric opacity & transparency. You may want to write down what I say about each range and transparency.