CK-12-Physics - Intermediate

http://www.ck12.org Chapter 23. Quantum Physics

As it turned out, this was key evidence that confirmed the ongoing work of French physicist Louis de Broglie (1892-
1987).

Waves and Particles

From the photoelectric effect and blackbody radiation, it was clear that light had unusual properties where it acted
as both a wave and a particle.

• It had frequency like a wave, and displayed effects of interference such as the two-slit experiment, as well as
  the Doppler Effect.


• At the atomic scale, however, it acted in some ways as a particle as in the photoelectric effect.


• What appeared at first to be a smooth variation of wavelengths was actually a very finely grained set of distinct
  wavelengths, or quanta, as shown in blackbody radiation.

This is known as the wave-particle nature of light, orwave-particle duality. Danish physicist Niels Bohr (1885-
1962) suggested that one must choose whether an experiment is to be performed using the wave nature of light or
the particle nature of light, but never both!

So, what is light really?

If modern physics teaches us anything, it is that the world of our senses is limited. Macroscopic phenomena such as
rolling balls and splashing waves appeal to our everyday senses and behave and accord to the expectations of those
senses. But in reality, the only things our senses are directly privy to are the net macroscopic effects of an untold
number of atomic interactions. We have no first-hand appreciation of how the atomic world behaves. And we have
no reason to assume that once we begin to get hints of this entirely different world, it would merrily conform to
our perceived macroscopic notions. Light, we may say, is that “thing” which manifests its behavior as a wave or
a particle. Perhaps, someday we will understand it as something altogether different, but probably not something
simpler.

The link below provides additional commentary on the nature of wave-particle duality.

http://demonstrations.wolfram.com/WaveParticleDualityInTheDoubleSlitExperiment/

Matter as waves

It is not unreasonable to pose the following question: If light can behave as a particle, is it not possible that particles
can behave as waves?

This was the very question that French physicist Louis de Broglie (1892-1987) asked in his doctoral thesis in 1925.
At the time, De Broglie’s side was received with skepticism, since there was no evidence which called for such an
explanation.

De Broglie’s thesis suggested that all material particles possessing momentum would have an associated wave.

We can show that a photon of light carries momentum:

p=Ec

The wavelength of the light wave can be found by replacingE=h fin the above equation:

p=

E

c

=

h f

c

=

h f

λf

=

h

λ

→

p=

h

λ

Equation A