Light The only thing we see! buckleyc/light.htm.
Transcript of Light The only thing we see! buckleyc/light.htm.
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Light
The only thing we see!
http://www.newi.ac.uk/
buckleyc/light.htm
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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
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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
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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
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Io’s Orbit
Io and Jupiter
Earth and Sun
Diameter of earth’s orbit: 300,000,000 km
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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
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Michelson’s Experimental Design
Michelson Animation
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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
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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)
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Electromagnetic Spectrum
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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?
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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
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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?
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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
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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
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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.
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ShadowsShadows• 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
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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
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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
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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
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Applications of Polarizing Filters
• Sun Glasses– reduce glare– block out half of the light
• 3-D movies