Chapter 27: Light. Conceptual Physics Hewitt, 1999 Bloom High School. 27.1 Early Concepts of Light. Ancient Greeks thought that particles of light traveled from your eye to the object Socrates & Plato Argued (from ancient Greece to Renaissance) that light was a particle or a wave
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Chapter 27: Light Conceptual Physics Hewitt, 1999 Bloom High School
27.1 Early Concepts of Light • Ancient Greeks thought that particles of light traveled from your eye to the object • Socrates & Plato • Argued (from ancient Greece to Renaissance) that light was a particle or a wave • Wave 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 Light • Originally 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 hours • When 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 year • Light 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 space • 8 light minutes to get light from the Sun • 4 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 Waves • Electromagnetic wave- atoms emit partially electrical and partially magnetic energy, we see it as light • Electromagnetic spectrum- the full range of electrical and magnetic radiation • Infrared- waves longer than visible light • Heat • Ultraviolet- waves shorter than visible • Causes sunburns
27.4 Light and Transparent Materials • Forced vibration- a non-vibrating object that vibrates as a result of other vibrations • Works with light as well as sound • Transparent- wavelength(s) of light causes the object to vibrate in the same manner • All absorbed light is re-emitted • Glass, water, transparencies, etc. • Can be transparent to some wavelengths and not to others
27.5 Opaque Materials • Opaque- absorption of light without full re-emission • Wood, stone, people, metals • Our atmosphere is opaque to high-frequency UV (luckily)
27.6 Shadows • Ray- a way of describing light travelling in a straight line • Shadow- 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 Polarization • Polarization- changing fully rotating light waves into a single plane of radiation • Sunglasses, storefront and windshield glazing • With a second filter, light can be blocked
27.8 Polarization & 3D Viewing • The 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?