Diffraction produces fringes by causing light waves to spread out and overlap, leading to interference patterns of light and dark bands.
When light encounters an obstacle or passes through a narrow opening (like a slit), it doesn't just cast a sharp shadow; it bends and spreads around the edges. This phenomenon is called diffraction. Think of waves in water spreading out after passing through a gap in a barrier.
The Role of Diffraction and Interference
Fringes are the result of the interplay between diffraction and interference:
- Diffraction: As light passes through one or more slits, each slit essentially acts as a new source of light waves spreading outwards in all directions.
- Interference: When these waves from different sources (the slits) overlap, they interact with each other. This interaction is called interference.
- Constructive Interference: If the peaks of one wave meet the peaks of another wave, or troughs meet troughs, their amplitudes add up, resulting in a brighter spot.
- Destructive Interference: If the peaks of one wave meet the troughs of another wave, they cancel each other out, resulting in a darker spot.
These patterns of constructive and destructive interference create the alternating bright and dark bands we see as fringes on a screen placed some distance away from the slits.
Diffraction Gratings and Sharper Fringes
The principles of diffraction and interference are particularly clear with a diffraction grating, which consists of an array of many identical, equally-spaced slits. According to the reference, when light encounters a diffraction grating:
- The bright fringes, which come from constructive interference of the light waves from different slits, are found at specific, predictable angles.
- These angles are the same as those found with just two slits.
- However, the pattern produced by a diffraction grating is much sharper and brighter compared to a pattern produced by only two slits. This is because constructive interference occurs more strongly and destructively in the regions between the bright fringes when waves from a large number of slits combine.
Summary Table: Interference and Fringe Formation
| Interference Type | Wave Interaction | Result on Screen | Fringe Type |
|---|---|---|---|
| Constructive | Peaks meet Peaks, Troughs meet Troughs | Amplified Brightness | Bright Fringe |
| Destructive | Peaks meet Troughs | Cancel Out | Dark Fringe |
In essence, diffraction provides the multiple, spreading waves, and interference is the mechanism by which these waves combine to form the distinct pattern of bright and dark fringes.
