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Physics scientists since the discovery of these phenomena. In this context, it is interesting to note what Richard Feynman has sa*d in his famous Feynman Lectures on Physics: No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them. The best we can do is, roughly speaking, is to say that when there are only a few sources, say two interfering sources, then the result is usually called interference, but if there is a large number of them, it seems that the word diffraction is more often used. In the double-slit experiment, we must note that the pattern on the screen is actually a superposition of single-slit diffraction from each slit or hole, and the double-slit interference pattern. 10.6.2 Seeing the single slit diffraction pattern It is surprisingly easy to see the single-slit diffraction pattern for oneself. The equipment needed can be found in most homes –– two razor blades and one clear glass electric bulb preferably with a straight filament. One has to hold the two blades so that the edges are parallel and have a narrow slit in between. This is easily done with the thumb and forefingers (Fig. 10.16). Keep the slit parallel to the filament, right in front of the eye. Use spectacles if you normally do. With slight adjustment of the width of the slit and the parallelism of the edges, the pattern should be seen with its bright and dark bands. Since the position of all the bands (except the central one) depends on wavelength, they will show some colours. Using a filter for red or blue will make the fringes clearer. With both filters available, the wider fringes for red compared to blue FIGURE 10.16 can be seen. Holding two blades to In this experiment, the filament plays the role of the first slit S in form a single slit. A bulb filament viewed Fig. 10.15. The lens of the eye focuses the pattern on the screen (the through this shows retina of the eye). clear diffraction With some effort, one can cut a double slit in an aluminium foil with bands. a blade. The bulb filament can be viewed as before to repeat Young’s experiment.Indaytime,thereisanothersuitablebrightsourcesubtending a small angle at the eye. This is the reflection of the Sun in any shiny convex surface (e.g., a cycle bell). Do not try direct sunlight – it can damage the eye and will not give fringes anyway as the Sun subtends an angle of (1/2)°. In interference and diffraction, light energy is redistributed. If it reduces in one region, producing a dark fringe, it increases in another region, producing a bright fringe. There is no gain or loss of energy, which is consistent with the principle of conservation of energy. * Richard Feynman was one of the recipients of the 1965 Nobel Prize in Physics 268 for his fundamental work in quantum electrodynamics. Reprint 2026-27