Light behaves like a wave, and that has consequences
Light exhibits wave behavior — it has a wavelength, and when multiple light waves overlap, they combine according to the same rules as any other wave. Two key consequences of this are interference and diffraction, and both produce effects that are hard to explain if you only think of light as traveling in perfectly straight rays.
Interference: waves adding and canceling
When two light waves overlap, they combine by superposition: where two wave crests line up, they add together for a brighter result (constructive interference); where a crest lines up with a trough, they cancel out (destructive interference).
You’ve likely seen this without realizing it: the swirling rainbow colors on a soap bubble or a thin film of oil on water come from light reflecting off both the front and back surfaces of the thin film, then interfering with itself. Because different wavelengths (colors) interfere constructively at slightly different film thicknesses, you see shifting bands of color rather than a single reflected color.
Diffraction: bending around obstacles
Diffraction is the tendency of waves to bend and spread out around obstacles and through openings, especially when the obstacle or opening is a similar size to the wave’s wavelength. Light passing through an extremely narrow slit doesn’t produce one sharp bright line on a screen — it spreads into a pattern of bright and dark bands, because different parts of the wave passing through the slit interfere with each other on the way to the screen.
This is also why shadows aren’t perfectly sharp if you look closely: light bends slightly around every edge, blurring the boundary between light and shadow, even though the effect is usually too subtle to notice with the naked eye.
Diffraction gratings: interference and diffraction working together
A diffraction grating — a surface with thousands of closely spaced parallel lines — uses these effects deliberately to split light into its component wavelengths, similar to a prism but through interference rather than refraction. This is the working principle behind spectrometers, instruments astronomers use to determine what distant stars are made of, and how fast they’re moving (through the same redshift effect covered in this site’s Research section).
Why this matters
Interference and diffraction are two of the clearest everyday demonstrations that light is fundamentally a wave phenomenon — and they’re also the same underlying math used to understand X-ray crystallography (determining the structure of molecules, including DNA), noise-canceling headphones, and antireflective coatings on eyeglasses and camera lenses.