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

Io and Jupiter

Earth and Sun

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.

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

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