The two factors that affect diffraction are the wavelength of the wave and the size of the opening or obstacle it encounters. More specifically, what matters is the relationship between these two: diffraction becomes most noticeable when the wavelength and the opening (or obstacle) are similar in size.
Wavelength: Longer Waves Diffract More
Wavelength is the distance between one wave crest and the next. When a wave hits a barrier with a gap in it, longer wavelengths bend around the edges more dramatically than shorter ones. This is why you can hear someone talking around a corner even when you can’t see them. Sound waves from a human voice have wavelengths roughly 0.5 to 2 meters long, which bend easily around doorways and walls. Visible light, by contrast, has wavelengths around 400 to 700 nanometers (billionths of a meter), far too small relative to a doorway to produce any noticeable bending.
You can observe this effect even within a single type of wave. When a marching band plays on the street, low-pitched instruments are easier to hear from around a building corner than high-pitched ones. A 100 Hz bass note has a wavelength of about 3.45 meters, which bends readily around most obstacles. A 2,000 Hz note has a wavelength of only about 18 centimeters, so it tends to travel in a straighter line and gets blocked more easily. Distant thunder sounds like a low rumble for the same reason: only the long-wavelength, low-frequency components bend around buildings and terrain to reach you.
Opening or Obstacle Size: Smaller Gaps Spread Waves More
The second factor is the size of whatever the wave is passing through or around. For a gap (often called a slit or aperture in physics), a narrower opening produces more spreading. For an obstacle, a smaller object allows longer waves to wrap around it as though it weren’t there.
The math behind this is straightforward. For a circular opening with diameter D, the angle at which the wave spreads out follows the relationship θ = 1.22 λ/D, where λ is the wavelength. A smaller D means a larger angle, so the wave fans out more. A larger D means the wave passes through relatively straight, with less bending at the edges. The same logic applies to a rectangular slit: the first point where the wave cancels itself out (a dark fringe, in the case of light) appears at an angle given by sin θ = λ/a, where a is the slit width.
This is why a slightly cracked door lets sound spread freely into a room, filling the whole space, while light coming through the same crack appears as a narrow beam. The crack is many times larger than light’s wavelength but comparable to or smaller than sound’s wavelength.
The Ratio Between Them Is What Really Matters
Neither factor works in isolation. What determines the amount of diffraction is the ratio of wavelength to opening size (λ/D or λ/a). When that ratio is close to 1, meaning the wavelength and the gap are roughly the same size, diffraction is strongest and the wave spreads out dramatically. When the ratio is very small (wavelength much shorter than the opening), the wave passes through with minimal bending, behaving more like a straight beam. When the ratio is very large (wavelength much longer than the opening), the wave barely gets through at all.
This principle explains why scientists use X-rays, not visible light, to study the atomic structure of crystals. The spacing between atoms in a crystal is on the order of a few tenths of a nanometer. Visible light, with wavelengths hundreds of times larger than those gaps, cannot resolve such fine detail. X-rays have wavelengths short enough to match the atomic spacing, producing clear diffraction patterns that reveal how the atoms are arranged. As Richard Feynman noted in his physics lectures, if the spacing between atoms is smaller than the wavelength you’re using, you get no useful diffraction pattern at all.
Why This Sets Limits on What We Can See
Diffraction isn’t just a curiosity. It places a hard limit on how much detail any lens, mirror, or imaging system can capture. Every telescope, microscope, and camera has an aperture, and the formula θ = 1.22 λ/D governs the finest detail it can resolve. Two objects that are closer together than this angle will blur into one in the image, no matter how perfect the lens.
This is why optical microscopes cannot see viruses. A typical virus is 20 to 300 nanometers across, smaller than the shortest wavelength of visible light (about 400 nm). The light waves simply diffract around the virus without producing a sharp image. To see something that small, you need a wave with a shorter wavelength, which is exactly what electron microscopes provide.
It’s also why large telescope mirrors produce sharper images. A bigger mirror means a larger D in the equation, which makes the diffraction angle smaller. That translates to finer detail. The same physics explains why radio telescopes, which detect waves with wavelengths measured in centimeters or meters, need to be enormous dishes (sometimes kilometers across in array form) to achieve any useful resolution.
How This Explains Everyday Wave Behavior
Once you understand these two factors, a lot of everyday wave behavior clicks into place. Ocean waves wrap around a jetty because their wavelengths (often tens of meters) are comparable to the width of the structure. Wi-Fi signals at 2.4 GHz have a wavelength of about 12 centimeters, which is why they bend around furniture and through doorways reasonably well, while higher-frequency 5 GHz signals (about 6 cm wavelength) are more easily blocked. FM radio waves, with wavelengths around 3 meters, diffract around buildings and hills far better than the millimeter waves used in some 5G networks.
In every case, the same two variables are at work: the wavelength of the wave and the size of whatever it’s interacting with. Change either one, and you change how much the wave bends.