dual nature crash course 12 th
DUAL NATURE OF LIGHT
Experimental phenomena of light reflection, refraction, interference, diffraction are explained only on the
basis of wave theory of light. These phenomena verify the wave nature of light.
Experimental phenomena of light photoelectric effect and Compton effect, pair production and pair annihilation
can be explained only on the basis of the particle nature of light. These phenomena verify the particle nature
of light. It is inferred that light does not have any definite nature, rather its nature depends on its experimental
phenomenon. This is known as the dual nature of light. The wave nature and particle nature both can not be
possible simultaneously
de-Broglie HYPOTHESIS
De Broglie imagined that as light possess both wave and particle nature, similarly matter must also posses both
nature, particle as well as wave.
De Broglie imagined that despite particle nature of matter, waves must also be associated with material particles.
Wave associated with material particles, are defined as matter waves
DAVISSON GERMER EXPERIMENT
The experimental arrangement is shown in figure. There are three main parts of this experiment
(i) Electron gun
Electrons of desired energy are produced in it by
the process of thermionic emission.
(ii) Nickle crystal diffracts the electrons beam obtained from
electron gun.Nickle crystal behaves like a three dimensional diffraction
grating
(iii) Detector (Ionisation chamber)
It detects the electron beam diffracted by the nickle crystal.
(iv) Conclusion and results
Curve between the intensity (I) of diffracted electrons and diffracting angle
In this experiment, on drawing different I–f curve intensity maxima is obtained at an angle of
diffraction of 500 and accelerating potential 54 volt.
From the differaction measurements wavelength associated with electron is obtained as 1.65 Å whereas
according to de Broglie theory this wavelength comes out to be 1.66 Å. Because both the results are same,
therefore we can say wave nature is associated with the moving electrons.
diffraction from crystal is studied using two equations called Bragg's equations. This are -
Where d = distance between two consequtive crystal plane
or interplanar distance.
D = Distance between two atoms in the same lattice plane
n = order of diffraction
l = De broglie wavelength associated with electron.
q = glancing angle
f = angle of diffraction
4. EXPLANATION OF BOHR QUANTISATION CONDITION
According to De Broglie electron revolves round the nucleus in the form
of stationary waves (i. e. wave packet) in the similar fashion as stationary
waves in a vibarting string. Electron revolves in those circular orbits whose
circumference is an integral multiple of de–Broglie wavelength associated
with the electron, 2pr = nl
* If amplitude of matter wave is y then y2(DV) = probability of finding in DV volume.
* Truely satisfactory physics of dual nature of matter has not developed so far. Quantum wave theory is still
subject of research.
*Observations on photoelectric effect imply that in the event of matter-light interaction, absorption of energy takes
place in discrete units of hv. This is not quite the same as saying that light consists of particles, each of energy hu.
* Observations on the stopping potential (its independence of intensity and dependence on frequency) are the
crucial discriminator between the wave-picture and photon-picture of photoelectric effect.
* The wavelength of a matter wave given by l =
h
p has physical significance; its phase velocity vp has no physical
significance. However, the group velocity of the matter wave is physically meaningful and equals the velocity of
the particle
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