PHYSICS 20A Lecture Notes - Lecture 9: Pendulum Clock, Roygbiv, Photon
Physics 20A – Lecture 9 – The Electromagnetic Spectrum
What is light?
• A wave
o Characterized by amplitude, wavelength, and frequency or period
• Light moves very quickly, but is not infinitely fast
o *If you want my opinion, light actually moves very slow relative to the size of the entire universe
(which is always expanding).
By comparison, we do not move at all
• c = 3×105km/s (not 8, this is in kilometers)
o distance between earth and moon 384,000km
o light takes 1.28s to traverse this distance
o c = 299,792,458m/s (Google result)
• How do we know the speed of light? (not by googling it)
o Galileo in 1667 – first recorded attempt to measure the speed of light
▪ Galileo & assistant in a big field with lanterns/lamps
▪ Stand away from each other at a known distance (1 mile for example; this lecture does
not reveal what distance they used)
▪ 1. Galileo uncovers his lamp
▪ 2. The assistant uncovers his lamp when he sees Galileo’s light
▪ 3. Galileo measures the elapsed time until he saw his assistant’s light
▪ v=d/t
▪ this didn’t work…
− likely used a pendulum clock (no stopwatches back then)
− found no difference in time
➢ b/c light is more than 10 times faster than sound
➢ speed of light = 186,000 miles/second
➢ at a distance of 1 mile
1mile/(186,000 miles/second) = 5×10-6 seconds
o Ole Römer (Danish astronomer) in 1676 – measured speed of light with Io (eye-o), a moon of
Jupiter
▪ Io was discovered by Galileo’s telescope
▪ When Io passes the shadow of Jupiter, it gets dark (disappears)
▪ Io circles Jupiter in a little less than 42.5 hours (it is Jupiter’s innermost moon)
▪ Römer measures how long it takes Io to go around the shadowed part of Jupiter
− To reappear after being eclipsed by Jupiter
− It took longer to reappear depending on where Earth was in its orbit
➢ Which means Earth’s distance to Jupiter changes over the year
➢ More distance for the light to traverse
➢ Light has a finite speed
− See Römer’s diagram on left
➢ As Earth moves toward Jupiter (F to G)
Io reappears from Jupiter’s shadow sooner
➢ As Earth moves away from Jupiter (L to K)
Io reappears from Jupiter’s shadow later
− Römer’s prediction, September 1676
➢ Announcement to members of the Paris Academie du Sciences
➢ Next expected eclipse of Io in November would occur exactly ten minutes
later than computed on the basis of previous eclipses
➢ Delay would arise from the fact that astronomers thought light travels
instantly
➢ He was right
➢ Then he said it would take 22 minutes for light to traverse the diameter of
Earth’s orbit around the sun
The correct answer is 16 minutes
That’s pretty good for 1676, though
Jupiter
& Io
Sun &
Earth
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Physics 20a lecture 9 the electromagnetic spectrum. The assistant uncovers his lamp when he sees galileo"s light: 3. Galileo measures the elapsed time until he saw his assistant"s light: v=d/t this didn"t work likely used a pendulum clock (no stopwatches back then) found no difference in time. B/c light is more than 10 times faster than sound. At a distance of 1 mile: ole r mer (danish astronomer) in 1676 measured speed of light with io (eye-o), a moon of. Io was discovered by galileo"s telescope: when io passes the shadow of jupiter, it gets dark (disappears, r mer measures how long it takes io to go around the shadowed part of jupiter. Io circles jupiter in a little less than 42. 5 hours (it is jupiter"s innermost moon) To reappear after being eclipsed by jupiter. It took longer to reappear depending on where earth was in its orbit. Which means earth"s distance to jupiter changes over the year.
