Fraunhofer vs. Fresnel Diffraction: Key Differences Explained
Fraunhofer diffraction occurs when light rays are parallel before and after interacting with an obstacle; Fresnel diffraction happens when either the source or the screen is close enough that the rays are still diverging or converging.
Phone-camera lens flares and CD rainbow streaks look similar, so engineers casually swap the names, thinking the math is interchangeable. The mix-up hides itself until the focus distance changes and the pattern suddenly shifts.
Key Differences
Fraunhofer: parallel light, far-field pattern, simpler math with Fourier transforms. Fresnel: curved wavefronts, near-field pattern, requires Fresnel integrals. Switching models without noticing the distance yields incorrect intensity maps and misaligned optical designs.
Which One Should You Choose?
Designing spectrometers or telescope gratings? Use Fraunhofer. Aligning laser cutters or smartphone lenses? Use Fresnel. Check the ratio of distance to aperture squared; if it’s ≫1, go Fraunhofer; if ≈1, stay Fresnel.
Examples and Daily Life
Shadows under a streetlamp show Fresnel rings when you stand close; step back and the edges sharpen into Fraunhofer silhouettes. Concert light shows exploit both: near-stage gobos create Fresnel fringes, while distant projectors deliver Fraunhofer beams.
Can I see Fresnel rings with my phone flashlight?
Yes—hold the light close to a pinhole in foil and project on paper inches away; you’ll see concentric rings.
Why do CD surfaces look like Fraunhofer gratings?
Your eye is far enough that the disc’s grooves act as a distant grating, producing parallel diffraction orders.
Which model is used in eyeglass prescriptions?
Optometrists use Fraunhofer-based wavefront analysis because the eye’s retina is optically distant from the lens.