1. What happens in a single slit setup
When light passes through a narrow slit, it spreads out and forms a pattern of bright and dark bands on a screen. This spreading is called diffraction. Even though there is only one slit, different parts of the same slit act like separate coherent sources and interfere with each other, producing the pattern.
2. Why a single slit creates a pattern
According to Huygens’ principle, every point along the slit behaves like a tiny source of wavelets. These wavelets overlap and interfere, and this interference forms the bright and dark regions.
The pattern is not like the regular equally spaced fringes in YDSE — the central bright region is much wider, and side regions get weaker quickly.
3. Appearance of the diffraction pattern
The diffraction pattern formed by a single slit has a very distinct look:
3.1. Central maximum
The central bright band is the widest and brightest. It is twice as wide as the next bright regions.
3.2. Secondary maxima and minima
On either side of the central maximum are smaller bright bands (secondary maxima). Between these bright regions lie dark bands (minima), where the light intensity falls nearly to zero.
4. Condition for dark fringes (minima)
Dark bands appear when the light from one half of the slit cancels the light from the other half. The condition for minima is:
\( a \sin \theta = n\lambda \)
where:
- \(a\) — slit width
- \(\theta\) — angle at which the minimum appears
- \(n = 1, 2, 3, ...\)
These points mark the dark bands on the pattern.
5. Condition for bright fringes (secondary maxima)
The positions of bright fringes are not given by a simple formula like in double slit interference. They occur roughly midway between the minima and become weaker as we move further from the centre.
6. Width of the central maximum
The angular width of the central bright band is:
\( \text{Angular width} = 2\theta_1 \)
where:
\( a \sin \theta_1 = \lambda \)
This shows that a narrower slit produces a wider central maximum because the diffraction becomes stronger.
7. Why the pattern is broader than double-slit fringes
In a double slit experiment, interference happens between two well-separated sources, producing narrow and evenly spaced fringes. In a single slit, interference happens across a continuous opening, so the central bright region is much wider and the side maxima get faint quickly.
8. Factors affecting the diffraction pattern
- Wavelength: Longer wavelengths produce wider spreading.
- Slit width: Narrower slits give stronger diffraction and wider maxima.
- Distance to screen: Increasing the distance makes the pattern easier to observe.
9. Everyday examples of single slit diffraction
The spreading of light through a small gap, the slight blurring of shadow edges, and the colourful patterns seen around very small openings are all examples of single slit diffraction in nature.
10. Why this proves the wave nature of light
If light behaved only as rays or particles, it would travel straight and form sharp shadows. The spreading and formation of alternating maxima and minima can only happen if light is a wave. Single slit diffraction is one of the strongest demonstrations of this.