Problem

When you cut a lens in half and separate the pieces, you create two coherent light sources—each half acts like a separate lens forming its own image of the point source. These two images become coherent secondary sources that interfere on the screen, much like a double-slit setup. The key is finding where each half-lens focuses the light (using the lens equation), then treating the separation between those image points as your effective "slit separation" in the interference formula $\beta = \frac{\lambda D}{d}$, where $D$ is the distance from the images to the screen.

When you cut a lens in half and separate the pieces, you create two coherent light sources—each half acts like a separate lens forming its own image of the point source. These two images become coherent secondary sources that interfere on the screen, much like a double-slit setup. The key is finding where each half-lens focuses the light (using the lens equation), then treating the separation between those image points as your effective "slit separation" in the interference formula $\beta = \frac{\lambda D}{d}$, where $D$ is the distance from the images to the screen.