The concept of light being both a wave and particle at the same time (a paradox) is the foundation quantum mechanics was built on. English scientist Thomas Young challenged the existing theory (Isaac Newton's) that light is a particle. In an effort to verify the properties of light, Thomas Young created the double-slit light experiment during the early 1800s.

"But what is light really? Is it a wave or a shower of photons? There seems no likelihood for forming a consistent description of the phenomena of light by a choice of only one of the two languages. It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do."

- Albert Einstein and Leopold Infeld The Evolution of Physics, pg 262-263

"Light and matter are both single entities, and the apparent duality arises in the limitations of our language."

- Heisenberg

Experiment

Cut one 2 inch by 2 inch square in the middle of a piece of thick paper (opaque, construction paper). This will be known as Paper A.

Repeat step 1 with a second piece of thick paper (opaque, construction paper). This will be known as Paper B.

Cut out 2 large squares of aluminum foil that are large enough to cover the squares in the thick paper. Tape the foil to each paper, covering both holes.

Use a razor blade to cut a 1 inch slit in the center of the aluminum foil on Paper A.

On Paper B, hold 2 razor blades together to cut 2 parallel slits (1 inch in length) in the center.







To keep the light on, a band (rubber band) is tightened around the laser's power button.



Shine the light through the slit in Paper A. The reflective side of aluminum foil should be away from laser pointer. Make note of the pattern on the surface behind Paper A.







Shine the light through the slit in Paper B. The reflective side of aluminum foil should be away from laser pointer. Make note of the pattern on the surface behind Paper B.







When light passes through a single slit, the beam shows properties of consisting of particles. The day-to-day proof that light appears to be a particle is in "photon counting". When intensity is reduced, individual photons can be observed. Many devices can be used in photon-counting. Photomultipliers are the most common. The number of photons observed per unit time is the photon flux.

When light passes through parallel slits, interference patterns are observed. Light is showing properties consistent with that of a wave supporting the paradox of a wave-particle duality. Interference patterns are the distinct bands of color separated by dark regions. Electrons also exhibit a similar wave-particle duality.