Wave-particle duality interactive: diffraction and polarisation with Malus' law
The only way to prove light is a wave
Particles travel in straight lines. Waves bend around corners and care which way they vibrate. Watch diffraction spread light through a narrowing slit, then rotate a polariser to prove transverse waves obey Malus' law: I = I₀ cos²θ. Drag the slider to change the angle.
Polarisation Experiment
0° Rotation
Rotate second polariser relative to first
Maximum transmission • I = I₀
Polariser angle θ
0°45°90°
Angle θ
0°
Intensity I/I₀
100%
When a wave hits an obstacle or slit comparable to its wavelength, it bends around the edges — this is diffraction. Narrower slits create wider spreading patterns because the wavefront is more constrained. When light passes through a single slit just a few wavelengths wide, it fans out into a characteristic interference pattern with a bright central maximum and dimmer side fringes. This spreading is impossible for particles, which would travel straight through and produce a sharp shadow. Double slits create the famous alternating bright and dark bands as waves from each slit interfere constructively and destructively — direct proof that light is a wave. Diffraction underpins everything from CD rainbow reflections to the resolution limits of microscopes and telescopes.
Know This
Diffraction proves waves because particles can't bend, and polarisation proves transverse oscillation because only waves with orientation can be filtered by a polarising plane.