![]() The diffraction pattern in the focal plane (the drawing is rather artistic, i.e., inaccurate) is easily observed for monochromatic light (a mercury lamp with a colour filter, or a laser), and can be modified (filtered) to modify the image. The first lens is used to construct a Fourier image in the focal plane, while the second lens forms back the image. 1, which shows how parallel light illuminates an object, and a symmetric set-up of two high-quality lenses. 1 detailed explanations of the mathematical description of imaging are provided. It turns out that a simple lens produces in its focal plane a diffraction pattern for the image. Physical optics makes use of the wave nature of light to understand these phenomena. While trying to take, e.g., a microscope to its limit, i.e., to improve the resolution, researchers in the middle of the 19th century (Abbé, in particular) found that the dark parts of the image inside an optical system contribute to the final image. Ray optics deals with the simple design of optical systems. The Fourier image is modified by cutting away low or high components (orders of the diffraction pattern) and a second lens is used to view the altered image. A HeNe laser is used to illuminate an image or a grating whose Fourier image is generated in the focal plane of a lens. In this experiment we investigate the diffractive (as opposed to refractive) properties of a lens, named Fraunhofer diffraction.
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