The path difference between two light waves directly determines the type of interference (constructive or destructive) and thus the appearance of interference fringes.
Here’s how it affects the pattern:
- Constructive Interference (Bright Fringes):
- Occurs when the path difference is an integer multiple of the wavelength (like one wavelength, two wavelengths, etc.).
- The waves arrive in phase, so their crests and troughs align, reinforcing each other.
- This produces bright fringes in the interference pattern.
- Destructive Interference (Dark Fringes):
- Happens when the path difference is an odd multiple of half the wavelength (like half, one and a half, etc.).
- The waves arrive out of phase, so a crest meets a trough, and they cancel out.
- This creates dark fringes.
- Changing Path Difference:
- As the path difference changes gradually across the screen, alternating regions of constructive and destructive interference occur.
- This forms a series of bright and dark fringes, spaced according to the amount of path difference at each point.
In essence, the pattern and position of interference fringes are entirely governed by the path difference between the interfering waves.