A7LAM CSCA ACADEMYCSCA Academy / Physics / Optics
CSCA Physics · Lesson 14 of 16Interference and diffraction
Interférences et diffraction · التداخل والحيود
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Light is a wave: two coherent beams add up into bright and dark fringes, and light spreads out when it passes through a small opening.
Key ideas
- Bright fringe where the path difference is kλ; dark fringe at (k + ½)λ.
- Double slit fringe spacing: Δx = Lλ/d.
- Diffraction is strong when the opening is about the size of λ or smaller.
- Thin films (soap bubbles, oil on water) show colours by interference.
Formulas to know
- Δx = Lλ / d
- Bright: δ = kλ
- Dark: δ = (k + ½)λ
Worked example
Double slit: d = 0.5 mm, L = 1 m, λ = 500 nm. Fringe spacing?
- Formula: Δx = Lλ/d
- Replace: Δx = 1 × 5×10⁻⁷ / 5×10⁻⁴
- Compute: Δx = 1 × 10⁻³ m
Answer: Δx = 1 mm
Practice questions
1. Red light replaces green light in a double slit. The fringes:
get closer · spread apart ✓ · disappear · stay the same
Red has a longer λ, so Δx is larger.
2. Path difference at the first dark fringe?
λ · λ/2 ✓ · 2λ · 0
(0 + ½)λ.
3. Which shows that light is a wave?
Reflection · Sharp shadows · Interference ✓ · The photoelectric effect
Only waves interfere.