Chapter 27: LightConceptual Physics

Hewitt, 1999Bloom High School

27.1 Early Concepts of LightAncient Greeks thought that particles of light

traveled from your eye to the objectSocrates & Plato

Argued (from ancient Greece to Renaissance) that light was a particle or a waveWave theory strengthened by light scattering

through diffraction (rainbow from a prism)Particle theory strengthened by light travelling in a

straight line and not spreading out like a wave Photons- massless light particles

27.2 The Speed of LightOriginally measured by Olaus Roemer (1675) by

measuring the orbital period of Io (Jupiter’s moon)When Earth was closer, the orbital period was 42.5

hoursWhen farther away , the period was longer!The change in Earths distance divided by the change

in observed period is the speed of light!300,000 m/s = c

Light yearLight year- the distance that light

travels in one year(300,000m/s)(60s/min)(60min/hr)

(24hr/day)(365dy/yr)9,460,800,000,000 m

Used as a standard distance in space8 light minutes to get light from the

Sun4 light years to the next closest star

(Alpha Centuri)100,000 light years from one end of

the Milky Way to the other

27.3 Electromagnetic WavesElectromagnetic wave- atoms emit partially

electrical and partially magnetic energy, we see it as light

Electromagnetic spectrum- the full range of electrical and magnetic radiationInfrared- waves longer than visible light

HeatUltraviolet- waves shorter than visible

Causes sunburns

27.4 Light and Transparent MaterialsForced vibration- a non-vibrating object that vibrates as a result of

other vibrationsWorks with light as well as sound

Transparent- wavelength(s) of light causes the object to vibrate in the same mannerAll absorbed light is re-emittedGlass, water, transparencies, etc.Can be transparent to some wavelengths and not to others

27.5 Opaque MaterialsOpaque- absorption of light without full re-

emissionWood, stone, people, metalsOur atmosphere is opaque to high-frequency UV

(luckily)

27.6 ShadowsRay- a way of describing light

travelling in a straight lineShadow- caused by rays being

blocked

Umbra- dark part in the middle of the shadow

Penumbra- lighter area on the edge of the shadow

Lunar Eclipse

27.7 PolarizationPolarization- changing fully rotating light

waves into a single plane of radiationSunglasses, storefront and windshield glazingWith a second filter, light can be blocked

27.8 Polarization & 3D ViewingThe movie is filmed

twice, simultaneously, with two polarizing films at 90° to each other

The glasses given separate out each “movie” taken

Without glasses, the combined movie appears blurry

Optical Illusions

Are the rows crooked?Animated gif or

not? Focus on the +What color are the dots?