The Two-Slit Experiment of Light
Two-slit light experiment by Young
Light clearly demonstrates wave-like properties based on this experiment. Bands of light and
darkness reflect amplification and inference of waves, respectively.

The Experiment
Slits S1 and S2 below behave as coherent sources of light waves that produce an interference
pattern on the viewing screen. The picture of the right represents an enlargement of the center of a
fringe pattern formed on the viewing screen.
The Wave/Particle Duality of Light
The results of Young's double-slit experiment, and Einstein's explanation of the photoelectric
effect, led physicists to the seemingly unavoidable conclusion that light is somehow both a wave
and particle at the same time.
However, light has never been observed to be both a wave and
particle at the same, only one or the other.
Source: Principles of Physics: Third Edition p. 1004
Source: Principles of Physics: Third Edition p. 1067
A three-dimensional rendition of the two-slit experiment
Additional renditions of the two-slit experiment
The Hypothetical Experiment: An Electron's "Wave-Particle" Duality Nature
Monoenergetic electrons in are projected towards a plate with two slits.

The Result
The electrons are detected as particles at a localized spot in some instant of time, but the
probability of arrival at that spot is determined by finding the intensity of two interfering waves.
An electron must, therefore, have a dualistic particle-wave nature.
Source: Principles of Physics: Third Edition p. 1066
Another Hypothetical Experiment: One Electron can Pass through Two Slits
Simultaneously

Beam of electrons are sent through with each slit closed half of the time.

The Result and Conclusion
The electrons show a clearly different diffraction pattern than when both slits are
open. The
Principles of Physics states: "Hence, our assumption that the electron is
localized and goes through only one slit when both slits are open must be wrong (a
painful conclusion!). "To interpret these results, we are forced to conclude that
an
electron interacts with both slits simultaneously
[Book's emphasis]. If we attempt
to determine experimentally which slit the electron goes through, the act of
measuring destroys the interference pattern. It is impossible to determine which
slit the electron goes through. In effect,
we can say only that the electron passes
through both slits!
The same arguments apply to photons.
Accumulated results of the two-slit electron diffraction experiment with each slit closed half of the
time. The result with both slits open is shown in red.
Conclusions of Modern Physicists:
Matter must a wave-particle nature to explain simultaneously Young's double-slit
experiment and Einstein's explanation of the photoelectric effect.

Matter and photons are able to be in two places at the same time.