Real double-slit patterns are more subtle than a simple series of equally bright bands. In practice, the interference fringes sit inside a broader brightness pattern that comes from diffraction at each slit.

The combined pattern

In a simplified model, a double slit produces a regular interference pattern of bright and dark bands. That picture is useful, but it is incomplete for real slits. Real slits have finite width, so each slit does not merely act as a single point source. Instead, each slit also produces single-slit diffraction, causing the light to spread out before the waves from the two slits overlap.

The observed pattern is therefore a combination of two wave effects:

• Double-slit interference, which creates the narrow bright and dark fringes

• Single-slit diffraction, which controls the overall brightness across the screen

The broad brightness pattern is called the envelope.

Intensity vs. angle for double-slit diffraction shown as three curves: a pure double-slit interference pattern (even-height peaks), a single-slit diffraction envelope (broad central hump), and the observed pattern (interference fringes multiplied by the envelope). This makes it visually clear that fringe spacing comes from slit separation, while the overall falloff in brightness comes from the finite slit width. Source

This means the double-slit maxima and minima are not all equally strong. Near the center of the screen, the fringes are usually bright and easy to see. Farther from the center, the fringes become dimmer because the diffraction envelope reduces the intensity there.

Experimental photographs comparing a single-slit diffraction pattern (broad central maximum with weaker side lobes) to a double-slit pattern (many fine fringes). The double-slit image shows the interference fringes concentrated in a bright central region and fading outward, consistent with modulation by a diffraction envelope. Source

The fringes are still part of the interference pattern, but their brightness is being shaped by diffraction.

What the screen pattern looks like

A correct description of the pattern includes both the fine structure and the broad structure at the same time.

• The screen shows many closely spaced bright and dark interference fringes

• Those fringes lie inside a much wider diffraction envelope

• The central region is usually the brightest part of the envelope

• Outer fringes are present, but their intensity decreases with distance from the center

• Some fringes may be extremely faint or not visible at all if the envelope suppresses them strongly

So, the actual pattern is not a set of identical bright bands.

Calculated double-slit intensity distribution showing rapid interference oscillations under a slowly varying diffraction envelope. The labeling ties the physics to the standard model $I(\theta)\propto \cos^2(\cdots),\mathrm{sinc}^2(\cdots)$, emphasizing that the observed pattern is the product of interference and diffraction. Source

It is a set of interference fringes whose intensities are modulated by the single-slit diffraction envelope.

Why the envelope appears

The envelope appears because each slit has width, and waves coming from different parts of the same slit can combine with one another. At some directions, the waves from different parts of one slit add strongly. At other directions, they partially or completely cancel. That produces the broad diffraction pattern from each slit.

Once that spreading has occurred, the light from one slit overlaps with the light from the other slit. The two overlapping waves then produce the familiar interference maxima and minima. The key idea is that the interference does not happen in isolation. It happens within the region where each slit has already sent light by diffraction.

Two levels of wave addition

This combined pattern can be understood as two layers of superposition.

First, within each individual slit, light from different parts of the slit combines and creates a diffraction distribution. That determines how much light from that slit is available at each angle.

Second, the waves from the two different slits combine with each other and create the alternating interference fringes.

Because of this two-step structure:

• diffraction determines the overall intensity trend

• interference determines the fine bright-dark spacing

A useful way to think about the pattern is that the diffraction envelope acts like a brightness guide for the interference fringes. Where the envelope is strong, the fringes are bright and visible. Where the envelope weakens, the fringes fade. Where the envelope reaches a diffraction minimum, an interference maximum can be suppressed so completely that a bright fringe may disappear.

Reading the pattern correctly

When you analyze a diagram or an experimental image, look for the broad central bright region first. That broad region is evidence of the single-slit diffraction envelope. Then look inside it for the narrower bright and dark bands, which show the double-slit interference.

This helps explain why textbook sketches that show evenly bright fringes are only partial models. A more realistic pattern has a strong center and weaker outer fringes. The brightest interference fringes are usually found near the center because the envelope is strongest there.

It is also important to distinguish between two kinds of dark regions:

• an interference minimum, caused by cancellation between waves from the two slits

• a diffraction minimum in the envelope, caused by cancellation among waves from different parts of the same slit

These are not the same idea, even though both appear as dark places on the screen. In a real double-slit pattern, both can affect what is seen.

Common misunderstandings

One common mistake is to think that diffraction and interference produce separate patterns in separate places. They do not. The interference fringes are superimposed within the diffraction envelope, so the observed pattern is one combined result.

Another mistake is to assume that every bright fringe should have the same intensity. That would only match an oversimplified picture that ignores slit width. Once real slit width is included, the envelope changes the brightness of the fringes across the screen.

A third misunderstanding is to treat the envelope as an extra physical object or barrier. It is not a material feature. It is simply the overall intensity pattern created by diffraction from each slit.

The essential idea for this subsubtopic is that a real double-slit pattern must include both effects at once: the narrow maxima and minima from interference and the broader envelope from single-slit diffraction that controls how strong those fringes appear.