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How does Youngâs double-slit experiment create light and dark fringes?
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When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.
Youngâs double-slit experiment demonstrates the wave nature of light through the principle of superposition. When light passes through two narrow, closely spaced slits, the resulting light waves overlap and interfere with one another on a distant screen, creating a pattern of alternating bright and dark fringes.12
The formation of these fringes is determined by the path difference between the waves originating from each slit:
- Constructive interference (Bright fringes): This occurs when the path difference between the two waves is an integral number of wavelengths (0, λ, 2λ, ...). At these points, the crests of the waves arrive simultaneously, reinforcing each other to produce high-intensity light.13
- Destructive interference (Dark fringes): This occurs when the path difference is a half-integral number of wavelengths (λ/2, 3λ/2, ...). At these points, the waves are out of phase, causing the crest of one wave to cancel the trough of the other, resulting in regions of complete darkness.123
For this pattern to be observable, the light sources must be coherent, maintaining a constant phase relationship.1