Two slits. The light from the two slits overlaps on the screen. Where the path difference is a whole number of wavelengths the waves add up (bright), and where it is a half-integer number they cancel (dark).

d sinθ = mλ (bright), d sinθ = (m + ½)λ (dark), Δy = λL/d

What changes the spacing. The fringes are further apart for a longer wavelength or a longer distance to the screen, and closer together for wider-spaced slits. The width of each slit does not change the spacing.

Single-slit envelope. Each slit is also wide enough to diffract the light, which spreads out the pattern. The double-slit fringes sit inside a broad envelope with its first minimum where a sinθ = λ. A narrower slit gives a wider envelope. The intensity is the product of the two effects: I = I₀ [sin(α)/α]² cos²(β), with α = πa sinθ/λ and β = πd sinθ/λ.

The screen shows a 10 cm wide strip, with the brightness made a little more generous than the graph, as your eyes would see it. The slits in the top picture are not to scale.