Diffraction: When Waves Bend
The Core Principles of Wave Bending
At its heart, diffraction is all about waves not traveling in perfectly straight lines. Imagine a group of marching soldiers who encounter a large tree. They would easily march around it in an organized way, reforming their lines on the other side. This is similar to how waves behave. When a wave meets a barrier, the parts of the wave that are not blocked become new sources of wavelets that spread out from that point. This idea is formalized in a concept called Huygens' Principle.
The most important factor that determines how much a wave diffracts is the size of the obstacle or gap compared to the wave's wavelength. The wavelength ($\lambda$) is the distance between two successive crests (or troughs) of a wave.
| Scenario | Amount of Diffraction | Real-World Example |
|---|---|---|
| Gap size is much larger than wavelength ($\lambda \ll$ gap) | Little to no spreading. The wave goes mostly straight through. | Light from a doorway creates a sharp shadow on the floor. |
| Gap size is similar to wavelength ($\lambda \approx$ gap) | Significant spreading. The wave bends strongly around the edges. | Water waves spreading out after passing through a harbor entrance. |
| Gap size is smaller than wavelength ($\lambda >$ gap) | Very strong spreading. The gap acts like a new point source. | Hearing low-pitched sounds (bass) from another room around a door. |
Diffraction in Different Types of Waves
Diffraction is a universal property of waves. It doesn't matter what kind of wave it is; if it's a wave, it will diffract. However, because different waves have vastly different wavelengths, we observe diffraction in different ways.
Sound Waves: Sound waves have wavelengths ranging from about 17 meters (for a 20 Hz bass note) to about 1.7 centimeters (for a 20,000 Hz treble note). A typical doorway is about 0.8 meters wide. For low-frequency (bass) sounds, the wavelength is larger than the door opening ($\lambda >$ gap), so they diffract much more easily. This is why you can hear the bass from your neighbor's party even when their door is closed, but the higher-pitched voices and melodies are muffled.
Water Waves: These are one of the easiest waves to observe diffracting. The wavelength of water ripples can be just a few centimeters. If you create water waves in a ripple tank and send them towards a barrier with a small gap, you will see the waves curve and spread out into the region behind the barrier. The smaller the gap, the more circular the waves become on the other side.
Light Waves: Light has extremely short wavelengths, from about 400 to 700 nanometers (billionths of a meter). Because these wavelengths are so much smaller than everyday objects like doors and windows, we don't usually see light bending around corners. However, if you look very closely at the edge of a shadow, you might notice it's slightly fuzzy and not razor-sharp. This fuzziness is due to diffraction. To see dramatic light diffraction, we need openings that are as small as light's wavelength, like the grooves on a CD or a finely scratched surface.
Seeing Diffraction in Action: A CD and a Laser Pointer
One of the most beautiful demonstrations of diffraction involves a simple CD or DVD and a laser pointer. The surface of a CD has a spiral track of tiny pits that are very close together, acting as a series of equally spaced gaps (a diffraction grating).
What to do: In a dimly lit room, shine a laser pointer at the surface of a CD (the shiny side, not the label side). Tilt the CD until you see a reflection of the laser dot. But look more carefully, and you will also see several other bright spots of light projected on the wall, often in different colors if you use a white light source instead of a laser.
What's happening: The closely spaced grooves on the CD are acting as multiple slits for the light to diffract through. Each groove diffracts the light, and these diffracted waves interfere with each other—they combine to create a pattern of bright and dark spots. This phenomenon is called diffraction grating interference. The different colors appear because white light is a mixture of all colors (wavelengths), and each wavelength is diffracted by a slightly different amount, spreading the light into a rainbow spectrum.
Common Mistakes and Important Questions
Q: Is diffraction the same as refraction?
Q: Why can't I see light bending around a building like sound does?
Q: Does diffraction only happen with a single slit or gap?
Footnote
[1] Aperture: An opening, hole, or gap. In the context of diffraction, it is the opening through which a wave passes.
[2] Wavelength ($\lambda$): The distance between two successive identical points on a wave, such as from crest to crest or trough to trough. It is usually measured in meters (m).
[3] Wavefront: An imaginary surface that connects all points of a wave that are in the same phase of vibration (e.g., all the crests). For a ripple from a single stone, the wavefront is a circle.