Where two coherent waves overlap, they reinforce in some places and cancel in others, painting bright and dark fringes. Measure the fringe spacing and you have measured the wavelength of light.
Interference is the superposition of waves from two coherent sources (constant phase difference, same frequency). Constructive interference (bright) occurs where the path difference is a whole number of wavelengths; destructive (dark) where it is an odd number of half wavelengths. For Young double-slit fringes, λ = ax / D, where a is the slit separation, x the fringe spacing and D the distance to the screen.
Change the slit separation, the wavelength and the screen distance and watch the fringe pattern respond exactly as λ = ax / D predicts.
Path difference decides what you see.
| Condition | Type | Result |
|---|---|---|
| path difference = nλ | constructive | bright fringe |
| path difference = (n + ½)λ | destructive | dark fringe |
| λ = ax / D | double-slit fringes | a = slit separation, x = fringe spacing, D = distance |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
Two wave sources are coherent if they have:
Constructive interference occurs where the path difference between the two waves is:
For Young double-slit fringes, the fringe spacing x is given by:
A dark fringe (destructive interference) occurs where the path difference is:
The double slit turns a length measurement into a wavelength.
The two sources must be coherent; two ordinary lamps will not interfere because their phase difference is random. In λ = ax / D, a is the slit separation and x the fringe spacing; swapping them inverts the answer. Constructive interference needs a whole number of wavelengths of path difference, while destructive needs an odd number of half wavelengths.
Unlocks once the checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
In a double-slit experiment the slit separation is 0.50 mm, the screen is 2.0 m away and the fringe spacing is 2.4 mm. The wavelength of the light is:
At a point on the screen the path difference from the two slits is 1.5 wavelengths. This point is:
To increase the fringe spacing in a double-slit experiment, you could:
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