SPM Physics: AHA! Diffraction vs Interference – Don’t Mix Them Up!

 

Students often confuse diffraction and interference, but examiners love to test this difference.

• Diffraction (one slit → spreading):
  • Light spreads out after passing through a narrow slit.
  • One slit only.
  • Wide central bright fringe.
  • Side fringes dimmer, uneven spacing.
  • Fringe pattern depends on slit width.

• Interference (two slits → overlapping):
  • Light from two coherent slits overlaps.
  • Two slits required.
  • Bright and dark fringes alternate.
  • Equal spacing, equal width.
  • Regular repeating pattern, nearly uniform brightness.

Quick comparison:

👉 Diffraction = spreading from one slit.

👉 Interference = overlapping from two slits.

Exam trap: Don’t write “both produce fringes” without details. State clearly: number of slits, fringe spacing, brightness, and pattern appearance.

AHA! Diffraction spreads, interference overlaps. Not by sight, but by INSIGHT!

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