Explore if Light Behaves as a Wave or a Particle

The Idea

Sir Isaac Newton was definitely no slouch when it came to physics. And, if you asked Newton whether light was a wave or a particle, he would say it was a particle. Newton was actually correct for reasons that would not become clear until several centuries later. Thomas Young proved the opposite was true—that light was (also) a wave. Today, we recognize that light has both particle and wave-like behavior. We re-create Young's experiment in this project to explore light's wavelike behavior. Young observed the effect of light emerging from two small slits in an opaque plate. Instead of two slits, you create a similar effect using a diffraction grating which lets you explore the effect of dozens of openings.

What You Need

• Diffraction grating, available from scientific supply companies
• Laser pointer
• Meter stick
• Ruler with metric divisions (centimeters)
• Index card
• Dark room
• Holders to support rulers
• Holders for card, diffraction grating, and grating

Method

Setting up

1. Mount the diffraction grating with the lines oriented up and down.
2. Mount the index card, so it is parallel to the diffraction grating.
3. Arrange the meterstick, so you can monitor the distance between the diffraction grating and the screen as you adjust this distance. (If it is convenient to set up, one possible approach is to attach the diffraction grating and screen directly to the meterstick, and then determine the distance between them from the difference in the readings.) A good starting distance is several centimeters.
4. Hold (or secure) the laser pointer, so the laser beam is directed perpendicular to the diffraction grating. See Figure 82-1.
5. Darken the room and turn on the laser. (Caution: As with any project involving a laser, use a low-power laser, such as a laser pointer, and be careful to avoid contact with anyone's eyes.)
6. You should see the path of the laser through the diffraction grating. The brightest spot is called the central maximum. Draw a vertical line through the central maximum.
7. On either side of the central maximum, you should also find a much dimmer spot. This is called the first order line. (It is more like a spot than a line in our case because we are using light from a laser, rather than the vertical slits originally used by Young.)

Expected Results

A pattern of spots is produced to the right and left of the center line. These are the result of the constructive interference of waves. This proves that light is a wave. Or, more accurately, in addition to having particle-like properties, light is also a wave.

If the distance to the screen is increased, the distance between the bright spots also increases. The distance between the laser and the diffraction grating should not matter, however, because the light strikes the diffraction grating in a perpendicular direction, regardless of how far it is coming from.

Why It Works

When waves meet, if the crests occur at the same time, the waves add. This is called constructive interference. If when waves meet a crest and trough come together, the waves cancel. This is called destructive interference.

Interference is a basic characteristic of waves. The light- and dark-spot pattern is the result of interference of the waves emerging from two adjacent openings in the diffraction grating.

Other Things to Try

Once you locate the first order bright spots, you can try to locate the second, third, and possibly higher order lines. This may require a very dark room.

The Point

This project recreates one of the most significant experiments of the twentieth century in which Thomas Young demonstrated that light is a wave. Interference patterns are a unique characteristic of waves. Because light in this experiment exhibits an interference pattern, it proves conclusively that light is a wave.

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