Lenses are used in Fraunhofer diffraction experiments primarily to focus the parallel light rays, which are characteristic of this type of diffraction, onto the observation screen.
Fraunhofer diffraction occurs when both the light source and the observation screen are effectively at an infinite distance from the diffracting object (like a slit or aperture). This effectively means that the wavefronts incident on the object are planar, and the diffracted wavefronts observed on the screen are also considered planar.
As stated in the reference: "Because in Fraunhofer diffraction, the source is at infinity so the rays of light which pass through the slit are parallel rays of light." This parallel nature of the light rays is a key characteristic of the Fraunhofer regime.
Role of the Converging Lens
While the theoretical framework of Fraunhofer diffraction assumes infinity, experiments are conducted in a lab. Lenses are crucial for simulating these infinite distances and making the observation practical.
The primary role of the lens, as highlighted in the reference, is related to focusing these parallel rays: "So in order to make these rays parallel to focus on the screen, we, make use of the converging lens."
- Incident Light: Often, a lens is placed before the diffracting object to make the light from a finite source (like a lamp) parallel, simulating a source at infinity.
- Diffracted Light: A second converging lens is placed after the diffracting object. The parallel rays diffracted by the object are then converged by this lens to form the diffraction pattern on a screen placed at the focal plane of the lens. This effectively brings the "infinity" of the diffracted wavefronts into a manageable focal point on the screen.
In summary, the lens serves to:
- Create parallel light incident on the diffracting aperture (simulating source at infinity).
- Focus the diffracted parallel rays onto an observation screen (bringing the pattern from "infinity" to a finite, observable location).
This setup allows the characteristic Fraunhofer diffraction pattern, which is formed by the interference of parallel rays, to be clearly observed and measured on a screen within a laboratory setting.
