Physics · Ch 7 — Wave Optics

Experimental Set Up for Fraunhofer Diffraction

7.9.2

Experimental Set Up for Fraunhofer Diffraction

The standard laboratory setup for observing Fraunhofer diffraction (Fig. 7.14) uses two converging lenses. A monochromatic point (or, for slit-shaped elements, linear) source S is placed exactly at the focal point of the FIRST lens; light emerging from this lens, ignoring aberrations, consists of parallel rays -- a genuine plane wavefront -- travelling towards the diffracting element itself (which could be a single slit, a circular aperture, a double slit, or a diffraction grating). After diffracting at this element, the emerging beam (now spread over a range of diffraction angles) passes through a SECOND converging lens, which focuses parallel rays leaving the element at any one particular angle down to a single corresponding point on a screen placed at that second lens's focal plane.

Figure 7.14Experimental set-up for Fraunhofer diffraction — a monochromatic source at the focus of a lens gives a plane wavefront on the slit, and a second lens focuses the diffracted parallel rays on the screen

$Fig. 7.14 — Experimental set-up for Fraunhofer diffraction — a monochromatic source at the focus of a lens gives a plane wavefront on the slit, and a second lens focuses the diffracted parallel rays on the screen$

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this figure shows. A monochromatic source S at the focus of a converging lens produces a plane wavefront that falls on the diffracting slit. The diffracted parallel rays are brought to a focus on the screen by a second converging lens (F ≈ D, D >> a) — the arr …

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