A light review Our primary source of light

A light review: • Our primary source of light is the sun. • Light travels in straight lines at a speed of 3. 00 x 108 m/s. • Light energy from the sun travels through space , reaches earth, and some of it turns to heat energy and warms the earth’s air. • Light from the sun also travels to the cells of green plants (producers) and is stored as energy. The peak wavelength of our sun is green. • When light reaches an object, it is absorbed/ reflected (giving it colour), or passes through it.

EM Theory • Henry and Faraday linked electricity and magnetism by finding that changing magnetic fields will generate a current in nearby conductors. • The linking of these two forces (electric and magnetic) allowed physicists to move closer to the ultimate goal of linking all the forces into a Grand Unified Theory (G. U. T. ) • The nuclear forces (weak and strong) are explained by an interaction of EM waves • Gravity is the elusive force that has not been nailed down into the G. U. T.

EM Theory • In 1846, Faraday theorized that light could be a vibration of electric and magnetic fields (dropping the ether model) • James Maxwell proved Faraday’s theory mathematically, 10 years later. • All electromagnetic waves are generated by accelerated charges. • As a charge is accelerated, it will continually emit excess energy as an EM wave (to limit the particle’s velocity to less than “c”). • The accelerated charge generates a changing magnetic field which creates a changing electric field, which generates a changing magnetic field,

EM Theory

What is light really? Electromagnetic radiation waves • Light waves are three dimensional • Light waves vibrate in all planes around a center line. • The waves have high points called “crests. ” • Waves also have low points called “troughs. ” • *The distance from one crest to the next crest is called a “wavelength. ” • *The number of waves passing a given point in one second is called the “frequency. ” wavelength

*Electromagnetic Radiation • Electromagnetic radiation can be described in terms of a stream of photons. Each photon is traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy. • The only difference amongst radio waves, visible light, and gamma-rays is the amount of energy of the photons. Radio and AC waves have photons with low energies. Microwaves have a little more energy than radio waves. Infrared has still more energy. • As we move down the chart we see visible, ultraviolet, X-rays, and gamma-rays whose photon energies gradually increase. • Gamma and Cosmic rays have the highest energy waves.

Remember radio waves are long…and gamma rays are small. Radio-TV -Microwave- Infrared - VISIBLE -Ultraviolet -X-rays - Gamma- Cosmic

EM Theory • An electron forced to oscillate at 102. 1 MHz will produce an FM radio wave • An electron accelerated by a Mega-Volt and fired into a metal target will produce X-rays (rapid deceleration of electrons) • A magnetron in a microwave oven accelerates charges to produce an EM wave that is forced to oscillate at about 2. 4 x 109 Hz (the electric field spins polar water molecules to create heat).

Radio (Longest electromagnetic waves) • Emitted by – Astronomical Objects – Radio Station Transmitters • Detected by – Ground based radio telescopes – *If you turn on a radio, , it will convert the radio wave energy into sound energy.

Television • Shorter than radio, also used to carry messages (pictures & sound) to our TV sets. • We can detect the TV waves around us with our televisions.

Microwave • Emitted by: – Gas clouds collapsing into stars – Microwave Ovens – Radar Stations – Cell Phones • Detected by – – Microwave Telescopes Food (heated) Cell phones Radar (systems)

Infrared (Heat or Thermal) Are you a source of infrared? YES you are! • Emitted by – Sun and stars (Near) – TV Remote Controls – Food Warming Lights (Thermal) – *Everything at room temperature or above, =HEAT • Detected by – Infrared Cameras – TVs, VCRs, – Your skin

Visible Each color is a different size wave. Red the longest & violet the shortest • Emitted by – The sun and other astronomical objects – Laser pointers – Light bulbs • Detected by – Cameras (film or digital) – Human eyes – Plants (red light) – Telescopes

Ultraviolet Sunburn / black light • Emitted by – – Tanning booths (A) The sun (A) Black light bulbs (B) UV lamps • Detected by – Space based UV detectors – UV Cameras – Flying insects (flies)

• Emitted by – – – X-ray Astronomical objects X-ray machines CAT scan machines Older televisions Radioactive minerals Airport luggage scanners • Detected by – Space based X-ray detectors – X-ray film – CCD detectors

Chandra X-ray Observatory *Slide from 2004 NSTA Conference • Chandra is designed to observe X-rays from high energy regions of the universe, such as the remnants of exploded stars. • The most sophisticated observatory built to date. • Deployed by the Space Shuttle Columbia on July 23, 1999,

Gamma Ray (Short electromagnetic waves but more energetic) • Emitted by – Radioactive materials – Exploding nuclear weapons – Gamma-ray bursts – Solar flares • Detected by • --Geiger counters – Gamma detectors and astronomical satellites – Medical imaging detectors

Sources of g-ray Emission • Black holes • Active Galaxies • Pulsars • Diffuse emission • Supernovae • Gamma-ray bursts • Unidentified

COSMIC Rays (The highest energy waves and the deadliest) • Cosmic rays come from deep space and can pass through the Earth. • Neutrinos are emitted by all stars • Dark matter/energy?

• A great question! • Radio waves= (Buildings to human size); • Microwaves (Humans-beetles); • Infrared waves (Eye of a needle); Visible waves (microscopic size)! All the rest are the size of molecules, atomic nuclei and smaller. .

EM Waves • In 1865, Maxwell proved that all EMR travels at the speed of light where: • The permittivity of free space is εo = 8. 85 x 10 -12 C 2/(N●m 2) and the permeability of free space is μo = 4π x 10 -7 T●m/A • The speed of light is defined today as c = 299 792 458 m/s

EM Waves • EM waves are produced using the following steps: • Energy is supplied to a source of EM radiation • The source does work on charges making them accelerate • The charges continually emit EMR radiating away excess kinetic energy • The atomic model proved a challenge as accelerated electrons (in the Bohr-orbit model) did not radiate EMR continuously (the atom would collapse) and Bohr solved this by stating this didn’t occur in the atom.