Introduction to Single-Slit Diffraction
Have you ever noticed that shadows are rarely perfectly sharp at their edges? Or that light passing through a tiny opening spreads out rather than travelling in a straight line? This behaviour is called diffraction , a fundamental property of all waves.
Diffraction: The spreading of a wave as it passes through an opening or around an obstacle, most pronounced when the size of the opening is comparable to the wavelength of the wave.
When light passes through a narrow slit, the waves spread out and interfere with each other, producing a characteristic diffraction pattern on a screen , a bright central band flanked by alternating dark and bright regions of decreasing intensity. This pattern is direct evidence that light behaves as a wave.
Think of water waves approaching a narrow gap in a harbour wall. If the gap is much wider than the wavelength, the waves pass through with little bending. But if the gap is about the same size as the wavelength, the waves fan out in all directions on the other side. Light behaves the same way.
The key factors that determine the diffraction pattern are:
- The wavelength of the light,
- The slit width,
Diffraction is most significant when . When , diffraction is negligible and the light travels essentially in a straight line, casting a sharp shadow.
Huygens' Principle and the Origin of the Pattern
To understand why a slit produces a diffraction pattern, we use Huygens' Principle.
Huygens' Principle: Every point on a wavefront can be treated as a source of secondary spherical wavelets. The new wavefront is the envelope (superposition) of all these secondary wavelets.
When a wavefront arrives at a single slit, every point across the width of the slit acts as a new point source of secondary waves. These secondary waves spread out and interfere with one another:
- Where secondary waves arrive in phase, they constructively interfere → bright regions (maxima)
- Where secondary waves arrive out of phase, they destructively interfere → dark regions (minima)
The result is the characteristic single-slit diffraction pattern:
- A bright, wide central maximum at the centre
- Dark minima on either side
- Weaker secondary maxima between the minima, decreasing in intensity further from the centre
The central maximum is significantly brighter and wider than any of the secondary maxima. The first secondary maximum has an intensity of only about 4.5% of the central maximum's intensity.
