Transcript Light in General

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