Key Concepts

You may have heard that light consists of particles called photons. How could something as simple as light be made of particles? Physicists describe light as both a particle and a wave. In fact, light's wavelike behavior is responsible for a lot of its cool effects, such as the iridescent colors produced on the surface of bubbles. To see a dramatic and mind-bending example of how light behaves like a wave, all you need is three pieces of mechanical pencil lead, a laser pointer and a dark room.

Sound is a great example of a wave that propagates, or travels, much like ripples in a pond do. In both cases kinetic energy flows through matter without permanently displacing the molecules in the matter itself—instead, it puts the matter through phases of compression (where the molecules get pushed together) and rarefaction (where the molecules spread apart). Think of the inside of a speaker vibrating with the music.

When waves come into contact with one another, they exhibit interference: waves that are all in phase (rarefying or compressing the same particles at the same time) add together to become stronger, and waves that are out of phase with one another (for example, one wave attempts to rarefy particles in a medium while another attempts to compress those same particles) cancel out. This is how noise-canceling headphones work—they produce a sound wave that resembles the wave responsible for the unwanted sound, but with the original phases of rarefaction and compression flipped. This has the effect of dampening the offending sound wave's effect on the air molecules. So that by the time its energy reaches your ear, the sound you perceive is more of a whisper than a shout—or an airplane engine's roar is more like a quiet hum.

Diffraction is another important feature of waves: When waves encounter small openings, they spread out after they pass through. In the following experiment we'll set up two slits to give waves of light the opportunity to diffract as they travel through them. The different points at which the diffracted waves overlap should demonstrate some cool patterns of constructive and destructive interference, and you'll get to witness the puzzling effect of light "canceling itself out."

• Three or more pieces of mechanical pencil lead (either 0.5 or 0.7 millimeter)
• Laser pointer (Red will work just fine, but green produces a more dramatic effect.)
• Dark room

• Darken the room.
• Stand about four feet from a wall.
• Hold the three pieces of pencil lead between your thumb and forefinger of your nondominant hand (your left hand if you are right-handed). Spread them apart very slightly so that you create two tiny gaps between the leads. These will act as your diffraction slits.
• Shine your laser through the slits you created, and look at the pattern of light emitted on the wall. What do you see?
• Play around with the positioning of your pencil leads, your laser and the width of the slits you created. When you get everything right, you should see a distinct pattern of dots appear on the wall behind the pencil lead.
Extra: Try using more pencil leads to create more diffraction slits. How does adding more slits change the pattern of light projected on the wall?

Results and observations
Laser light is emitted in the form of parallel waves that are coherent, or in phase with one another: all of the peaks and valleys line up. This is quite different than the light emitted from a flashlight—the rays are neither parallel nor in phase with each other. The laser's waves diffract as they pass through the slits you made, fanning out in a shotgunlike pattern from each slit. This allows them to interfere with one another as they overlap. In some places the waves will interfere constructively, creating bright spots on the wall. In other places the waves cancel themselves out, leading to the dark spaces you see between the spots.

If light demonstrated particlelike behavior exclusively, you would see only two dots on the wall corresponding to the locations of the slits. Oddly enough, Isaac Newton understood light this way: as a stream of particles, like a series of baseballs being thrown in a straight line. The problem posed by the double-slit experiment is that "baseballs" thrown through one hole seem to care about what the baseballs thrown through the other hole do! In the 19th century scientists decided that light must be a wave, but after witnessing light demonstrating particlelike behavior, Albert Einstein proposed that light can indeed be described as a particle (called a photon). The physicist Max Planck panicked, claiming, "the theory of light would be thrown back not by decades, but by centuries" if the scientific community were to accept Einstein's theory! But scientists ultimately arrived at the conclusion that light is both a particle (photon) and a wave.

Think of light's wave function as corresponding to the likelihood of a photon being in a certain place at a certain time. This makes it a little easier to understand how photons are forced to arrive at certain positions on the wall when their waves interfere with one another. What's less intuitive is the fact that photons fired one at a time toward two slits still demonstrate the same wavelike interference behavior after they pass through—it's as though individual photons are able to travel through two slits at once while still arriving at one location!

More to Explore
The Original Double-Slit Experiment, from Veritasium
Double-Slit Experiment Explained! By Jim Al-Khalili, from The Royal Institution
Wave–Particle Duality of Light, from
Young's Double-Slit Experiment, from YouTube
Bubble Colors, from The Exploratorium

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