Chapter 27: Light Conceptual Physics Hewitt, 1999 Bloom High School
Dec 26, 2015
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
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