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Light is an ELECTROMAGNETIC WAVE
Light is also a PARTICLE: the PHOTON
This nominal contradiction is an example of
COMPLEMENTARITY or DUALITY in QUANTUM MECHANICS. Depending on circumstances it is preferable to use one or the other point of view for light, electrons, protons, atoms: anything which is too small to be described by normal physics and directly experience requires using quantum mechanics
wavelength x frequency = speed of light = constant
v = c = 2.9979x108 m s-1 = 3.00x1010 cm s-1
= 3.00x105 km s-1= 186,000 miles/s
The speed of light does not depend upon direction or frequency in a vacuum and doesn’t need a medium. It is a CONSTANT of NATURE.
Microwave or millimeter between Radio and IR
Mirror reflects light in a particular direction
Movie screen scatters light in all directions
Interactions between light and matter determine the appearance of everything around us: objects reflect some wavelengths, absorb others and emit others.
(where metals emit electrons when light shines on them)
(where only specific wavelengths of light emerge from particular elements)
Or, max=2,900,000/T (nm)
This is the PEAK WAVELENGTH for BLACKBODY (or Thermal, or Planckian) emission from a SOLID, a LIQUID or a DENSE GAS.
=500 nm = 5000Å
Wien's Law Applet
Hotter objects emit more light at all frequencies per unit area.
Hotter objects emit photons with a higher average energy.
Since T can be measured by Wien's Law and L can be obtained from the star's or planet’s brightness and distance (we'll discuss later) this formula lets astronomers find the SIZES of planets and stars!
We’ll do that later; for now let’s compare luminosities:
In words, Planet 1 has 4 times the luminosity of Planet 2
Nucleus size only around 10-15m while electron clouds are roughly 10-10m = 0.1 nm = 1Å
As nearly all of the mass is in the nucleus, matter is mostly empty space!