10.6. DIFFRACTION

If we look clearly at the shadow cast by an opaque object, close to the region of geometrical shadow, there are alternate dark and bright regions just like in interference. This happens due to the phenomenon of diffraction. Diffraction is a general characteristic exhibited by all types of waves, be it sound waves, light waves, water waves or matter waves.

Diffraction refers to the bending and spreading of waves when they encounter obstacles or openings. When observing the shadow of an object, instead of a sharp boundary between light and dark, one will notice a pattern of alternating light and dark areas. This occurs because waves (like light) can bend around corners and spread out after passing through narrow slits or around obstacles. Such behavior is not limited to light; it applies to all types of waves, including sound and water waves.

Imagine the way sound travels around a corner. If someone is speaking in another room, you can still hear them even though you can't see them. This is similar to diffraction: the sound waves spread and bend around the corner, reaching your ears even though the direct path is blocked.

In the discussion of Young’s experiment, we stated that a single narrow slit acts as a new source from which light spreads out. Even before Young, early experimenters – including Newton – had noticed that light spreads out from narrow holes and slits. It seems to turn around corners and enter regions where we would expect a shadow.

A narrow slit allows light to pass through and then spread outwards. This is analogous to what happens when waves in water pass through a narrow gap. After passing through the slit, light behaves as if it is emanating from new, virtual sources along the width of the slit, and this causes a broad pattern of light with a central bright area and alternating dark and bright fringes on a screen placed to catch the light.

Think of how a garden hose works. If you partially cover the end of a hose, the water sprays out and spreads out in a cone-shaped fountain pattern. Similarly, light goes through a slit like the hose and spreads out into a broader area, creating patterns of light and shadow.

When the double slit in Young’s experiment is replaced by a single narrow slit (illuminated by a monochromatic source), a broad pattern with a central bright region is seen. On both sides, there are alternate dark and bright regions, the intensity becoming weaker away from the centre.

When light passes through a single slit, it diffracts and creates a pattern of intensity that has a central maximum (the brightest spot) and then decreases in brightness in adjacent areas. This pattern results from the interference of light waves emanating from different parts of the slit. The light waves interfere with one another, leading to areas of constructive and destructive interference. The central maximum is the brightest because it receives direct light and contributions from all parts of the slit.

Think of a flashlight being shone through a narrow opening. At close range, the light is concentrated and bright, but as you move further away, the light disperses more, creating a gradient from bright to dim. The brighter areas correspond to where the light waves constructively interfere, just like the bright central region of the diffraction pattern.

There has been prolonged discussion about the difference between interference and diffraction among scientists since the discovery of these phenomena. In this context, it is interesting to note what Richard Feynman has said in his famous Feynman Lectures on Physics: 'No one has ever been able to define the difference between interference and diffraction satisfactorily.'

The discussion around interference and diffraction often involves overlapping concepts. Both phenomena arise from the wave nature of light, but interference typically involves the superposition of waves from two or more sources, while diffraction refers to the spreading of waves as they encounter obstacles or openings. Richard Feynman's observation highlights that while we can distinguish these terms based on the context, they fundamentally describe similar wave behaviors.

Consider waves in a pond. If you drop two stones in at different points, the overlapping waves create interference patterns where they overlap. However, if you drop a stone and observe the ripples spreading out from a corner of the pond, that's diffraction. Both are wave behaviors but are perceived differently based on the situation.

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.

To observe diffraction, one can create a simple experiment using household items. By holding two razor blades with a narrow slit between them and positioning them in front of a light source like a bulb, the light will pass through the slit and produce a visible diffraction pattern, revealing the bright and dark bands characteristic of the phenomenon.

Imagine setting up a tiny stage for a puppet show using a narrow opening. The light from a single bulb highlights the puppet but also casts intricate shadows behind it. Like that puppet show, when light passes through the narrow slit between the blades, it reveals patterns that play out visually across a surface, making the invisible behavior of light waves visible.

--