double-slit interference
Shine light through two very fine, closely spaced slits and let it land on a screen. You might expect two bright lines, one behind each slit. Instead you get a whole row of alternating bright and dark bands, evenly spaced, spilling out to either side. This striped pattern is double-slit interference, and it answers a profound question: is light a wave? The stripes say yes, unmistakably.
The reason is superposition, the rule that when two waves overlap, their disturbances add. Light from the two slits spreads out and overlaps on the screen. At points where the two paths differ by a whole number of wavelengths, the crests line up with crests, the waves reinforce, and you get a bright fringe (constructive interference). At points where the paths differ by a half wavelength (or any half-odd number), a crest meets a trough, the waves cancel, and you get a dark fringe (destructive interference). The bright fringes fall where the path difference d sin(theta) = m lambda, with d the slit separation, theta the angle from the centre, lambda the wavelength, and m an integer (0, 1, 2, ...) called the order. The fringes are brightest in the centre and evenly spaced.
First done by Thomas Young around 1801, this experiment settled a long debate by showing light behaves as a wave, and the fringe spacing even lets you measure the wavelength of light. An honest and startling caveat: send the light through one particle, one photon, at a time, and over many photons the same striped pattern still builds up, each photon somehow interferes with itself. That is a doorway into quantum mechanics and wave-particle duality: light is neither a simple wave nor a simple stream of bullets, but something that shows wave behaviour here and particle behaviour elsewhere.
Two slits 0.20 mm apart are lit by red light (lambda = 650 nm) and the screen is 2.0 m away. The first bright fringe sits at sin(theta) = lambda/d = 650e-9/0.20e-3 = 0.00325, giving a fringe about 6.5 mm from the centre, easily visible.
Bright fringes where d sin(theta) = m lambda; the wave nature of light made visible.
The pattern needs coherent light (a single wavelength, steady phase), like a laser, which is why two separate light bulbs never make fringes. Strikingly, the fringes still form one photon at a time, a hallmark of quantum wave-particle duality.