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)(24 hr/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

Lunar Eclipse

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?