Double-Slit Experiment

Intermediate

One of the most famous experiments in physics, demonstrating the wave nature of light.

Double-Slit Experiment Setup
Light passes through two slits and creates an interference pattern
Experiment Parameters
Calculated Values
Fringe Spacing
2.75 mm
Δy = λL/d
Wavelength
550 nm
Green light
d/λ Ratio
364
Separation ratio
Angular Width
0.16°
First minimum

💡 Experiment Tips:

  • Wavelength: Change color to see how different wavelengths affect fringe spacing
  • Slit separation: Smaller separation = wider fringes
  • Screen distance: Further screen = larger, more spread out pattern
  • Slit width: Affects the envelope (single-slit diffraction) that modulates the pattern
  • Central bright fringe is always at the center (zero path difference)
  • The pattern demonstrates that light behaves as a wave!

Theory

Young's Double-Slit Experiment

When coherent light passes through two narrow slits, it creates an interference pattern of bright and dark fringes on a screen. This demonstrates that light behaves as a wave.

Path Difference: δ = d sin(θ)

Bright Fringes: δ = nλ (n = 0, ±1, ±2, ...)

Dark Fringes: δ = (n + ½)λ

Fringe Spacing: Δy = λL / d

Key Concepts

Coherent Sources

The two slits act as coherent sources - they maintain a constant phase relationship because they come from the same light source.

Interference Pattern

Alternating bright and dark fringes appear due to constructive and destructive interference of waves from the two slits.

Path Difference

The difference in distance traveled by light from each slit determines whether interference is constructive or destructive.

Central Maximum

The brightest fringe at the center where path difference is zero and waves arrive in phase.

Parameters Explained

  • Wavelength (λ): Smaller wavelengths (blue) create tighter fringe spacing
  • Slit Separation (d): Larger separation creates tighter fringes
  • Screen Distance (L): Greater distance spreads out the pattern
  • Slit Width: Affects the single-slit diffraction envelope

Historical Significance

Thomas Young performed this experiment in 1801, providing compelling evidence for the wave theory of light. It challenged Newton's particle theory and revolutionized our understanding of light. Later, the experiment would play a crucial role in quantum mechanics by demonstrating wave-particle duality.

Applications

  • Wavelength measurement - Precisely determine wavelength of light sources
  • Optical testing - Check quality of lenses and mirrors using interferometry
  • Spectroscopy - Diffraction gratings (many slits) separate light by wavelength
  • Holography - Creates 3D images using interference patterns
  • Quantum mechanics - Demonstrates wave-particle duality with electrons and photons
  • Laser characterization - Measure coherence length and beam quality

Discussion