1. What diffraction means
Diffraction is the bending and spreading of light when it passes around edges or through narrow openings. Even though ray optics predicts sharp shadows, the wave nature of light allows it to bend slightly into the shadow region.
This spreading becomes noticeable only when the size of the opening or obstacle is comparable to the wavelength of light.
2. Why diffraction happens
Diffraction is a direct result of the wave nature of light. According to Huygens’ principle, every point on a wavefront acts as a source of new wavelets. When part of a wavefront is blocked by an obstacle, the remaining edge continues to send out wavelets, causing the wave to bend.
3. Where diffraction becomes significant
Light has a very small wavelength (around 400–700 nm). So diffraction effects become visible only when openings or edges are extremely small — usually a fraction of a millimetre.
3.1. Large openings vs small openings
- Large opening (>> wavelength): Little or no spreading; shadow edges remain sharp.
- Opening comparable to wavelength: Significant spreading; diffraction patterns appear.
4. Diffraction patterns
When light diffracts through a narrow slit, the wave spreads out and forms a pattern of bright and dark regions on a screen. This happens because the different parts of the wave interfere with each other after spreading.
4.1. Central maximum
The central bright region is the widest and brightest part of the diffraction pattern. Its width depends on the wavelength and the slit width.
4.2. Side maxima and minima
On either side of the central bright region, there are alternating bright and dark bands called secondary maxima and minima. These are caused by interference between wavelets emerging from different parts of the slit.
5. Mathematical condition for minima
The dark fringes (minima) in diffraction occur when the path difference between rays from the edges of the slit leads to destructive interference. The condition for minima is:
\( a \sin \theta = n\lambda \)
Here:
- \(a\) = slit width
- \(\theta\) = angle of diffraction
- \(n = 1, 2, 3, ...\)
These angles mark positions of dark bands.
6. Why diffraction differs from interference
Interference involves two or more separate coherent sources. Diffraction, however, can happen with a single slit because different parts of the same slit behave like separate sources and interfere with each other.
So diffraction is often described as “interference within a single wavefront”.
7. Factors affecting diffraction
- Wavelength: Longer wavelengths diffract more.
- Slit width: Narrower slits produce more spreading.
- Distance to the screen: A larger distance makes the pattern easier to observe.
8. Everyday examples of diffraction
Even without lab setups, diffraction can be seen in many places:
- Sound bending around obstacles (sound has large wavelength).
- Light spreading slightly around the edges of a door.
- Colour patterns on CDs and DVDs (due to tiny grooves acting like slits).
- The halo around lights on a foggy night (droplets causing scattering + diffraction).
9. Diffraction proves the wave nature of light
If light behaved only like particles or straight rays, it would never bend into the shadow region. The systematic spreading and the formation of alternating bright and dark bands occur only because light is a wave.
This is one of the strongest confirmations of wave optics.