What you are looking at
A wave of light arrives from the left and strikes a barrier with
two narrow slits. By
Huygens' principle each slit acts as a fresh source of circular waves. As these two sets of waves
spread into the region on the right they
overlap and interfere — and where they meet on
the screen you see alternating
bright and dark fringes. The glowing field in the middle
shows the time-averaged wave intensity; the bright strip on the right is the pattern on the screen, with
its intensity plotted as the curve.
Constructive and destructive interference
At a point on the screen the two waves have travelled slightly different distances. When that
path difference is a whole number of wavelengths the crests line up and add — a
bright fringe. When it is a half-wavelength out, crest meets trough and they cancel — a
dark fringe. This gives the bright-fringe condition:
d sin θ = m λ (m = 0, ±1, ±2, …)
For a screen a distance L away, the fringes are evenly spaced by
Δy = λ L / d
so a
longer wavelength or a
smaller slit separation spreads the fringes
further apart. Slide the wavelength from violet to red and watch the bands widen.
The single-slit envelope
Each slit by itself also diffracts, producing a broad bright central region that fades toward the edges.
That is why the fringes are not all equally bright — they sit under a
diffraction envelope
set by the slit width a. The number of bright fringes packed under the central envelope is roughly
d / a, so widening the slits (larger a) leaves fewer visible fringes. Make a as wide as
d and you are heading toward a single broad slit — explore that in the separate single-slit experiment.
Things to try
Change the
wavelength and watch the fringe spacing and colour change together. Increase
the
slit separation to crowd the fringes closer; increase the
slit width
to narrow the envelope and dim the outer fringes. This is the experiment Thomas Young performed in 1801,
and the same pattern appears even when particles — electrons, atoms, whole molecules — are sent through
one at a time, which is one of the deepest puzzles in quantum mechanics.