Chapter 12 : Wave paticle Duality
1)
Classical physics connot explain many phenomena particularly those in
which light (or em radiation) interacts with electrons or atoms.
2)
Light or any em radiation consists of a huge collection of photons,
each photon having energy h,
where h is Planck's constant and
is the frequency.
3) An evidence for photons is the
photoelectric effect) where photocurrent depends on the inensity of
incident light as long as the frequency is greater than a critical
value c.
But if the frequency is less than c
, no photo current flows. The kinetic energy of the electron freed by
the photoelectric effect depends on the frequency not on the light
intensity.
4) Photon has a mass, a linear momentum and a
constant speed which is the speed of light. It has a size denoted by
the wavelength. If a photon falls on a wall, it applies a small force
on it, but if it falls on an electron, the electron will be thrown
off due to its small mass and size.
5) Compton effect proves
the particle nature of photons, where a photon has mass, speed and
linear momentum.
6) A wave describes the collective behavior of
photons.
7) The wave length of a photon is Planck's constant
divided by the linear momentum. The same relation applies to a free
particle, where the wavelength describes the wave nature of the
particle, i.e., the wave accompanying the particle.
8) The
electron microsocpe proves de Broglie's relation for particles. It is
used to detect diminutive particles.
9) Quantum mechanics is
based on assumptions which agree with experimental observations, for
the cases when an electron is trapped within limited confienmet.
While classical physics applies when the electron is free to move or
when the contining size is extensive.
Basic
Laws::
1- E=
h
E: energy of photon, h planck's constant.
h = 6.626 x
10-34
Js,
frequency Hz
Photoelectric Effect: Einstein proposed the
following:
The energy needed to free an electron from a metal
surface (called the work fucntion) is Ew
= E= hc
c
is the threshold (critical value) of the frequency, of the incident
photon.
b- If the photon energy exceeds this limit, the
electron is freed and the energy difference (h
- Ew)
is carried by the electron as KE.
c- If h
< Ew,
the electron would not be emitted at all, no matter how intense the
light might be.
d- c
and Ew
vary for different materials and do not depend on the light
intensity, the exposure time, or the voltage difference between the
anode and the cathode.
3- Photon properties:
a-
Photon is a concnetrated packet of energy which has mass, velocity
and linear momentum.
b- Its enegry E = h
c- It always moves at the speed of light C = 3 x108
m/s, regardless of its frequency.
d- The photon has a mass of
h
/ C2,
while in motion.
e- The photon linear momentus is h
/ C
= PL
4- Einstein showed that mass and energy are equivalent E = mC2.
5-The force which a beam of photns applies to the surface is
the change in linear momentum per second:
6-
Where
Pw
in the power is watt of the light incident on the surface. This force
is appreciable if it affect a free electron, so it throws it off.
This is the explanation of compton
effect.
10-
Wave properties of matter:
De Broglie put a hypothesis of wave
particle duality to particles, the wave length of a prticle must be
in analogy with a photon
11-
