Lecture #6: Thursday, Jan. 26
Solar eclipse: Sun and moon have roughly the same angular size, therefore
the moon can sometimes cover the sun. The earth is then in the shadow of the moon (picture)
Phase of the moon ?
The orbit of the moon is inclined by 5 degree to the ecliptic. There is not an eclipse every month.
Lunar eclipse: The moon is in the shadow of the earth. (same picture)
Phase of the moon ?
- Wavelength (l) : Distance from crest to crest
- Amplitude Height of the crest (from baseline)
- Frequency: Number of crests that passes an observer per second
Unit: Hertz (Hz) 1 Hz = 1 crest / second
Period and Frequency (f) (Frequency = 1/ period )
- Wave speed Speed of a wave crest
- Wave Speed = wavelength times frequency (c=lf)
or in other words:
frequency = wave speed divided by wavelength (f=c/l)
- Light as a Wave
- Light is an electromagnetic wave. Electric and magnetic fields wave
perpendicular to each other. Light propagates "itself" and needs no medium.
(good for astronomy - it goes through vacuum)
- Wave speed: speed of light. Vacuum: 300,000 km/s. In medium slower !
(velocity = velocity in vacuum / index of refraction)
"Speed limit of the Universe"
- Amplitude squared = Intensity
- Frequency or wavelength = color
red : 700 nm (frequency = c/700nm = 4.2e14 Hz)
yellow : 580 nm
green : 530 nm
blue : 450 nm
1 nm = 1 nano meter = 1e-9m or a millionth of a millimeter.
- Generation of light
- Waves are created if something moves up and down
- Light is created if a charged particle moves up and down
for example the electrons in matter
- What makes them move ? For example heat.
At T=0 K (= -273 degree C = -459 degree F) no motion ("absolute zero")
room temperature: 293K (20 degree C, 68 degree F): electrons move !
degree C = 5/9 * (degree F - 32)
- Why do stars emit light ? Because they are HOT !
- Hotter means brighter
Stefan-Boltzmann Law: Intensity = size * temperature4
- Light as a particle
- Sometimes, light behaves more like a particle stream
- Light comes in small energy packets called "Photons"
- Intensity = number of photons per second
- Frequency = energy of the photons
E = h * f with h as Planck constant: 4.136E-15 eV/s
(measure photon energy in eV (1 eV = 1.6 x 10-19 Joule))
(Joule is the SI unit for Energy: 1 Joule/s = 1 Watt, so for example a 100 Watt
light bulb emits 100 Joule/s)
use frequency = c / wavelength to get handy formula:
E (in eV) = 1240/wavelength in nm
(DEMO: photons in the classroom)
lower frequency, longer wavelength than visible light are:
basics, Laser Light through slits, 2 meter sticks