# lect 4 - The Nature of Light Guiding Questions How can...

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The Nature of Light

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Guiding Questions How can astronomers tell what distant celestial objects are made of? How can astronomers measure the temperatures of the Sun and stars? How can we tell if a star is approaching us or receding from us?
Determining the Speed of Light • Galileo tried unsuccessfully to determine the speed of light using an assistant with a lantern on a distant hilltop http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html

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Light travels through empty space at a speed of 300,000 km/s In 1676, Danish astronomer Olaus Rømer discovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth This happens because it takes varying times for light to travel the varying distance between Earth and Jupiter Using d=st with a known distance and a measured time gave the speed (s) of the light
In 1850 Fizeau and Foucalt also experimented with light by bouncing it off a rotating mirror and measuring time The light returned to its source at a slightly different position because the mirror has moved during the time light was traveling d=st again gave c

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Newton’s experiment on the nature of light
Is Light a wave or a particle? • Newton thought light was composed of particles • Young’s Double-Slit Experiment indicated light behaved as a wave

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Light is electromagnetic radiation In the 1860s, the Scottish mathematician and physicist James Clerk Maxwell succeeded in describing all the basic properties of electricity and magnetism in four equations This mathematical achievement demonstrated that electric and magnetic forces are really two aspects of the same phenomenon, which we now call electromagnetism
Because of its electric and magnetic properties, light is also called electromagnetic

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## This note was uploaded on 02/06/2011 for the course ASTR 113 taught by Professor Geller during the Spring '08 term at George Mason.

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lect 4 - The Nature of Light Guiding Questions How can...

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