Wave-particle duality interactive: diffraction and polarisation with Malus' law
The evidence that light is a wave
Waves bend around edges and care which way they vibrate. Narrow the slit to watch diffraction spread the light, then rotate a polariser to see transverse waves obey Malus' law: I = I₀ cos²θ. Change the slit width and the polariser angle and watch the pattern change.
Single-slit diffraction
Slit 4λ wide
Light of wavelength λ through one slit: brightness on a distant screen
Central band edges at ±14°
Slit width (in wavelengths)
1λ4λ7λ10λ
Slit width
4λ
First dark fringe θ
14°
Polarisation experiment
0° Rotation
Turn the axis of polariser P2 relative to P1. I₀ is the light reaching P2, already polarised by P1.
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. The classical particle model of light, with tiny bullets moving in straight lines, cannot explain this spreading or the fringes, which is the evidence that light behaves as a wave. Double slits create the famous alternating bright and dark bands as waves from each slit interfere constructively and destructively — strong evidence that light behaves as a wave. Diffraction underpins everything from CD rainbow reflections to the resolution limits of microscopes and telescopes.
Know This
Diffraction and interference are evidence for waves because the classical particle model of light cannot explain them, and polarisation shows transverse oscillation because only waves with orientation can be filtered by a polarising plane.