Huygen's principle is a useful concept to understand the propagation of waves, including the diffraction of light. According to Huygen's principle, every point on a wavefront can be considered as a source of secondary spherical wavelets that spread out in all directions. When these secondary wavelets overlap and interfere with each other, they determine the resulting pattern of the wave.
When a wave passes through a single slit, such as in a Young's double-slit experiment, Huygen's principle helps explain the resulting interference pattern. As the wave passes through the slit, each point on the wavefront becomes a source of secondary wavelets. These wavelets diffract and spread out, interfering with each other constructively and destructively, creating regions of constructive interference (bright fringes) and destructive interference (dark fringes) on a screen placed after the slit. This interference pattern is what we observe as an interference pattern in the case of a single slit.
However, before diffraction occurs, when the wave is not restricted by a narrow slit or obstacle, there is typically no noticeable interference pattern. The reason for this is that although every point on the wave can be considered as a source of secondary wavelets, these wavelets are spread out in all directions and do not interact in a coherent manner over large distances. The superposition of these wavelets does not result in a well-defined and observable interference pattern.
In the case of diffraction, where a wave encounters an obstacle or passes through a narrow opening, the wavefront becomes distorted, and the secondary wavelets from each point on the wavefront interact more coherently. This coherent interaction leads to the formation of distinct interference patterns that can be observed. The diffraction process helps to focus the wavefront and concentrate the interference effects, resulting in observable patterns on a screen or surface placed after the diffracting object.
In summary, while Huygen's principle explains the generation of interference patterns in general, it is the presence of diffraction that allows the wave to undergo coherent interactions, leading to observable interference patterns.