1. What Diffraction Means
Diffraction is the bending or spreading of waves when they meet an obstacle or pass through an opening. Instead of travelling straight, the wave ‘curves’ around the edges or fans out from the gap.
I like to think of diffraction as a wave trying to reach places it normally couldn’t — by bending around corners and spreading into new regions.
2. Definition of Diffraction
Definition: Diffraction is the phenomenon in which waves bend around obstacles or spread out when passing through narrow openings.
This behaviour is seen in water waves, sound waves, and even light waves (at very small scales).
3. Why Diffraction Occurs
Every point on a wavefront acts as a tiny source of new wavelets (according to Huygens' principle). When a part of the wavefront is blocked or restricted, the remaining wavelets spread into the region behind the obstacle.
This spreading makes the wave curve or fan out.
3.1. Huygens' Picture
Imagine the wavefront as made of many small ripple sources. If only a few ripple sources pass through an opening, they spread in all directions, creating a curved wavefront.
4. Diffraction Around an Obstacle
When a wave hits a barrier with an edge, the wave bends around that edge and enters the shadow region. The amount of bending depends on the wavelength.
4.1. Everyday Example
Sound bending around buildings is a clear example. Even if someone is talking behind a wall, the sound reaches you because sound waves diffract around the edges.
5. Diffraction Through an Opening
When waves pass through a narrow opening, the opening acts like a new source of waves. The waves spread out in a semicircular pattern after emerging.
5.1. Water Ripples Example
If you place a barrier with a small gap in a water tank and create ripples, the ripples spread out in circular arcs after passing through the gap. This is one of the easiest ways to see diffraction.
6. Effect of Wavelength on Diffraction
Diffraction depends strongly on the relation between the wavelength and the size of the opening or obstacle.
- Large wavelength → strong diffraction
- Small wavelength → weak diffraction
Waves spread the most when the opening size is close to the wavelength.
6.1. Examples
- Sound waves (long wavelength) diffract easily around corners.
- Light waves (tiny wavelength) diffract very little through large openings.
7. Diffraction of Sound Waves
Sound waves commonly show diffraction because their wavelengths are large (around a few centimetres to metres). That’s why sound can bend around corners easily.
7.1. Daily Observation
You can hear people talking even when you can’t see them because sound waves spread into the region behind obstacles.
8. Diffraction of Light Waves
Light also diffracts, but the effect is usually too small to notice in everyday life because its wavelength is extremely small (around 500 nm).
However, diffraction becomes visible when light passes through very narrow slits or tiny openings.
8.1. Example
Patterns of bright and dark fringes created in experiments using narrow slits are due to diffraction of light.
9. Applications and Importance of Diffraction
Diffraction explains many real-world wave behaviours:
- Why sound reaches you even when the source is not visible.
- How radio signals bend around buildings.
- Why water waves spread out behind obstacles.
- Why optical instruments use narrow slits to study wave properties.
Understanding diffraction helps in acoustic design, communication engineering, and experimental optics.