PHYSICS 20A Lecture Notes - Lecture 8: Electric Field, String Vibration, Astronomical Unit
Physics 20A – Lecture 8 – Light
More on Tides—Moon Phases and Tides
• Ex. If today’s a new moon, what time will there be a high tide?
o Noon, the sun is highest in the sky, crossing the meridian
Light and Radiation
• Light is a wave phenomenon – you must understand general wave properties
• Units of measurement
o Length: meter (m)
▪ 1m = 39.37 in
o Mass: kilogram (kg)
▪ 1kg = 2.2 lbs
o Time: second (s)
o Temperature: Kelvin (K)
▪ K = Celsius temperature + 273
▪ 0K = absolute zero – really frickin’ cold, man
o See Lecture 1 for a full list of prefixes
o Prefixes for small distances are important to measure optical or infrared wavelengths
▪ Ångstrom (Å)– a common astronomical unit; 1Å = 10-10m
• Waves
o Periodic disturbances going through some medium
▪ The medium does not travel with the disturbance
▪ Eg. Waves on water, sound waves in air, waves on a vibrating string, light waves, etc.
o Properties
▪ Amplitude (A) – height of a wave crest (the top of a wave)
▪ Wavelength () – distance between two successive wave crests or troughs (the bottom)
▪ Period (P) – time it takes for one complete wave cycle to pass a fixed spot
▪ Frequency (f) – number of wave crests passing a fixed point each second
− Unit: Hz (Hertz) = wave cycles per second
− One Hertz means one event of some kind per second
➢ i.e. 1 per second, Hz = 1/s
➢ f = 1/P
− e.g. P = 1 millisecond, f = ?
➢ f = 1/(1×10-3s)
1/(10-3s)
103Hz
1kHz
▪ Speed/velocity (v) – how fast a wave crest is traveling
− Waves with different wavelengths and frequencies can travel at the same speed
➢ Shorter wavelengths = more frequency, not faster
➢ Longer wavelengths = less frequency, not slower
− c = the speed of light in a vacuum = 3×108m/s
➢ a universal constant of nature
▪ (length per wave) × (waves per sec) = (length per sec) = speed of wave
− wavelength × frequency = speed
− for light in vacuum: f = c
➢ f = c/
wave crest
trough
amplitude
find more resources at oneclass.com
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Document Summary
More on tides moon phases and tides: ex. If today"s a new moon, what time will there be a high tide: noon, the sun is highest in the sky, crossing the meridian. Unit: hz (hertz) = wave cycles per second. One hertz means one event of some kind per second wave crest i. e. 1 per second, hz = 1/s f = 1/p. E. g. p = 1 millisecond, f = ? amplitude f = 1/(1 10-3s) 1khz trough: speed/velocity (v) how fast a wave crest is traveling. Waves with different wavelengths and frequencies can travel at the same speed. Shorter wavelengths = more frequency, not faster. Longer wavelengths = less frequency, not slower. C = the speed of light in a vacuum = 3 108m/s. A universal constant of nature (length per wave) (waves per sec) = (length per sec) = speed of wave. Wavelength frequency = speed for light in vacuum: (cid:540)f = c f = c/(cid:540)
