# How are Young's double slit experiment and speckle patterns related?

I know that when light from a laser is reflected from a rough surface onto a screen, it produces a speckle pattern.

I believe, a Fourier transform of the intensity distribution of the speckle pattern can be used to study the different interference patterns that make up the speckle pattern.

I was told that any two points on the rough surface separated by a distance $$d$$ contributes to the interference pattern that has a fringe spacing $$y=\frac{\lambda L}{d},$$ where $$\lambda$$ is the wavelength of the laser and $$L$$ is the distance from the rough surface to the screen.

However, because two points on any rough surface separated by a distance $$d$$ are also at different heights, say a difference in height of $$x$$, wouldn't the fringe spacing for two points points be $$y=\frac{(\lambda-x)L}{d}$$

In the image below, the red line illustrates the distance from the slit to the second point on the rough surface.