# 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

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

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 http://www.shep.net/physics/

How fast does light travel? Speed of light in a vacuum is constant in 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

Light Year Distance = velocity x time Light year = 300,000 km/s x 1 year = 300,000 km/s x 1 yr x 365 day/yr x 24 hr/day x 3600 s/hr Light year = 9.5 x 10 12 km

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)

EM Spectrum - speed 3x10 8 m/s

Sources of Light Luminous body –emits light waves Illuminated body –reflects light waves Rate of emitted light: luminous flux (P) –lumen (lm) Illumination of a surface: illuminance (E) –lm/m 2 or lux (lx)

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

Brightness Brightness (illuminance) is inversely related to distance squared Double the distance, the brightness is ¼

Light and Transparent Materials Light has a very high frequency –100 trillion times/second –10 14 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.

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 Transparent - light goes through in a straight line. Speed may be reduced. Opaque - light is absorbed by surface. 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

Color When light strikes an object it can be –transmitted –absorbed –reflected Color of the object is the color of the light reflected to your eye White - all light wavelengths are reflected Black - no light wavelengths are reflected

Primary colors of light –red –blue –green add together to make final color red, green, and blue light make white light http://www.explorescience.com/activities/Activity_page.cfm?ActivityI D=36

Primary Colors of Pigments Pigments absorb light. A pigment absorbs particular wavelengths of light Primary colors of pigments –yellow –cyan –magenta http://www.explorescience.com/activities/Activity_page.cfm?ActivityI D=37