The phenomenon of light fringes observed in both single-slit and double-slit experiments is indeed related to the interference of light, but the mechanisms behind the two cases differ. Understanding the distinction can help clarify why the double-slit experiment is typically attributed to interference.
In the case of a single slit, when light passes through it, the diffracted waves spread out and interfere with each other. This interference results in the appearance of a pattern of alternating bright and dark fringes on a screen placed behind the slit. This phenomenon is known as single-slit diffraction or the single-slit interference pattern.
The single-slit interference pattern arises due to the wave nature of light and can be explained using the principle of Huygens-Fresnel diffraction. According to this principle, every point on the slit acts as a source of secondary spherical waves, and these waves interfere with each other constructively or destructively, leading to the observed pattern.
On the other hand, in the double-slit experiment, light passes through two closely spaced slits, resulting in two separate diffracted waves. These waves then overlap and interfere with each other on a screen placed behind the slits. The interference pattern observed in the double-slit experiment is characterized by a series of bright and dark fringes, similar to the single-slit case but with additional properties.
The crucial distinction in the double-slit experiment is that the interference pattern cannot be solely explained by considering single-slit diffraction. The interference observed in the double-slit case is due to the coherent superposition of waves from the two slits. When the crests and troughs of the waves coincide, constructive interference occurs, resulting in bright fringes, whereas when they cancel out, destructive interference occurs, leading to dark fringes.
This distinct behavior of the double-slit experiment, where the interference pattern cannot be accounted for by single-slit diffraction alone, suggests that interference between waves from different slits is at play. This observation strongly supports the wave nature of light and has been extensively studied and confirmed experimentally.
It is worth noting that both single-slit and double-slit experiments involve wave phenomena and interference effects. However, the double-slit experiment is particularly compelling because it provides clear evidence of interference between waves from different sources (the two slits), which is not the case in the single-slit scenario.