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Light The only thing we see! http://www.newi.ac.uk/ buckleyc/light.htm Early Concept of Light • 500 BC – light is streamers emitted by the eye that make contact with the object (Socrates, Plato) • Pythagoreans from Greece believed that light traveled as particles to the eye • Other Greeks thought it traveled as waves • Einstein described massless particles of electromagnetic energy - photons Present Model of Light • Light has both particle and wave nature • Electromagnetic wave • A unit quantity of light is a photon http://www.walter-fendt.de/ph11e/emwave.htm Speed of Light • Roemer measured the time for Io (a moon) to orbit Jupiter in 1675 • The time varied depending on the position of earth’s orbit with the sun • When earth moving away from Jupiter, the period seem longer. • When earth moving toward Jupiter, period was shorter Light Speed Movie Io’s Orbit Earth and Sun Io and Jupiter Diameter of earth’s orbit: 300,000,000 km Michelson’s Experiment • Accurately measured the speed of light on earth in 1880 • first American to win the Nobel prize in 1907 • reflected light from a mirror 35 km away • spinning octagonal mirror allowed him to measure the time it took • 299,920 km/s 300,000 km/s Michelson’s Experimental Design Michelson Animation How fast does light travel? • Speed of light in a vacuum is constant in the universe • 7.5 round trips around the earth in one second • 8 minutes from the sun to the earth • 4 years from the nearest star, Alpha Centauri • 100,000 years to cross our galaxy • some galaxies are 10 billion light years away Electromagnetic Spectrum • Light is energy that is emitted by vibrating electric charges • called an electromagnetic wave • radio waves, microwaves, infrared, ultraviolet, x-rays, gamma rays are also electromagnetic waves • lowest frequency we see is red • highest frequency we see is violet (more energetic) Electromagnetic Spectrum Light and Transparent Materials • Light has a very high frequency – 100 trillion times/second – 1014 hz • Light hitting an object causes its electrons to vibrate • Result depends on the frequency of the light and the type of object – What types of results can occur? Transparent to Light • Transparent: lets light pass through in a straight line • Glass and water are transparent to light http://www.physicsclassroom.com/mmedia/waves/em.html Effect of frequency of light • Glass has a natural vibration frequency in the ultraviolet range • UV light hitting the glass causes a lot of vibration holding the energy within the glass • Glass does not transmit UV energy • Where does the energy go? Visible Light through glass • Visible light is transmitted by glass • The speed of light in glass is lower than in a vacuum. • Speed of light = c = 300,000 km/s • speed of light in glass = 0.69 c • speed of light in water = 0.75 c • speed of light in diamond = 0.40 c Opaque Materials • Absorb light and do not allow transmission • Metals are shiny because free electrons allow light energy to bounce back • Atmosphere is transparent to visible light and some infrared but opaque to most UV light • Clouds are transparent to UV rays When light hits an object • Transmission - light goes through a transparent object. Speed may be reduced. • Absorption - light is absorbed by surface on an opaque object. • Reflected - light bounces back off of surface • Some wavelengths (colors) of light may be absorbed while others are reflected giving the object color. Shadows • A shadow is formed when a light ray cannot reach a surface • sharp shadows – produced by small source close by – large source far away • total shadow: umbra • partial shadow: penumbra – light from another source fills in – large source only partially blocked Polarization • Light is a transverse wave • Light from most sources vibrates in all planes • Each light ray can be considered to have horizontal and vertical components • Separating vertical and horizontal components is called polarization Polarization • Polarizing filters are like sewer gratings that look like slits. • Light waves vibrating in the plane of the slit can make it through • Light waves that vibrate perpendicular to the grates cannot make it through • Polarization • A single polarizing filter will let about one half of the light through • Two polarizing filters aligned in the same direction will still let about one half of the light through • Two polarizing filters aligned perpendicular to one another will let almost no light through Applications of Polarizing Filters • Sun Glasses – reduce glare – block out half of the light • 3-D movies