Thursday, 9 July 2009

Planck's constant

Planck's constant:
In the early 1900's, German physicist E. Planck noticed fatal flaw in our physics by
demonstrating that the electron in orbit around the nucleus accelerates. Acceleration
means a changing electric field (the electron has charge), when means photons should
be emitted. But, then the electron would lose energy and fall into the nucleus.
Therefore, atoms shouldn't exist!

To resolve this problem, Planck made a wild assumption that energy, at the sub-atomic
level, can only be transfered in small units, called quanta. Due to his insight, we call
this unit Planck's constant (h). The word quantum derives from quantity and refers to a
small packet of action or process, the smallest unit of either that can be associated with
a single event in the microscopic world.

Changes of energy, such as the transition of an electron from one orbit to another
around the nucleus of an atom, is done in discrete quanta. Quanta are not divisible and
the term quantum leap refers to the abrupt movement from one discrete energy level to
another, with no smooth transition. There is no ``inbetween''.
The quantization, or ``jumpiness'' of action as depicted in quantum physics differs
sharply from classical physics which represented motion as smooth, continuous change.
Quantization limits the energy to be transfered to photons and resolves the UV
catastrophe problem.
Wave-Particle Dualism:
The wave-like nature of light explains most of its properties:
l reflection/refraction
l diffraction/interference
l Doppler effect
But, the results from stellar spectroscopy (emission and absorption spectra) can only be
explained if light has a particle nature as shown by Bohr's atom and the photon
description of light.

This dualism to the nature of light is best demonstrated by the photoelectric effect,
where a weak UV light produces a current flow (releases electrons) but a strong red
light does not release electrons no matter how intense the red light.

Einstein explained the photoelectric effect by assuming that light exists in a
particle-like state, packets of energy (quanta) called photons. There is no current flow
for red light because the packets of energy carried by each individual red photons are
too weak to knock the electrons off the atoms no matter how many red photons you
beamed onto the cathode. But the individual UV photons were each strong enough to
release the electron and cause a current flow.
It is one of the strange, but fundamental, concepts in modern physics that light has both
a wave and particle state (but not at the same time), called wave-particle dualism.

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