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PhysicsOpticsIntermediateStarter track

Concept module

Diffraction

Watch a wave spread after one narrow opening, see why diffraction grows when wavelength competes with slit width, and build the wave-optics bridge toward double-slit interference.

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Starter track

Step 2 of 50 / 5 complete

Wave Optics

Earlier steps still set up Diffraction.

1. Polarization2. Diffraction3. Double-Slit Interference4. Dispersion / Refractive Index and Color+1 more steps

Previous step: Polarization.

Start from track beginning

Why it behaves this way

Explanation

Diffraction is the spreading of a wave after it passes through a narrow opening or around an edge. A wide opening lets the outgoing wave stay relatively straight, but once the opening width and wavelength become comparable the wave no longer stays tightly collimated.

This page keeps the geometry intentionally bounded: one slit, one screen, and one movable probe. The same slit width, wavelength, and probe height drive the stage, the pattern graph, prediction mode, worked examples, and challenge checks so the pattern stays tied to one honest wave-optics story instead of a giant optics lab.

Key ideas

01Diffraction becomes more noticeable when the wavelength is large relative to the slit width, so the central peak spreads out instead of staying narrow.
02The screen pattern is not random blur. It follows a single-slit envelope with clear bright and dim regions set by the edge-to-edge path difference across the opening.
03A double-slit pattern still depends on diffraction because each slit has its own spreading envelope before the two slits interfere with one another.

Frozen walkthrough

Step through the frozen example

Frozen walkthrough
These examples read the current slit geometry directly from the live state, so the algebra stays attached to the same pattern you are seeing on the screen.

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Frozen valuesUsing frozen parameters

For the current slit width a = 2.4 and wavelength lambda = 1, where should the first diffraction minimum appear?

Slit width

2.4 m

Wavelength

1 m

Wavelength-to-slit ratio

0.42

1. Start from the first-minimum condition

Use the single-slit condition .

2. Substitute the live opening and wavelength

.

3. Read the geometric consequence

.

First-minimum result

The first minimum sits about 25.06^\circ from the center, so the central peak spans roughly 5.05 m on the screen.

Spread-width checkpoint

You want the central diffraction peak to spread out without moving the screen. Which control change is the most reliable move?

Make a prediction before you reveal the next step.

Choose whether you should change the wavelength, the slit width, or the probe position.

Check your reasoning against the live bench.

Increase the wavelength or decrease the slit width so the ratio lambda / a gets larger.
Probe position only samples a different part of the existing pattern. The actual spread changes when the wavelength becomes larger relative to the opening.

Common misconception

A narrower opening always makes the outgoing beam narrower because less of the wave gets through.

A narrower opening reduces the width of the source region, but it also increases the spreading of the outgoing wave.

That is why the central diffraction peak broadens when the opening gets smaller relative to the wavelength.

Quick test

Variable effect

Question 1 of 4

Answer from the live spread logic, not from isolated buzzwords.

Which change makes diffraction more noticeable for the same screen distance?

Use the live bench to test the result before moving on.

Accessibility

The simulation shows incoming plane wavefronts reaching a barrier with one vertical slit. On the right side, curved outgoing wavefronts spread from the opening toward a screen strip that brightens or dims according to the diffraction intensity.

A movable probe marks one screen point. Optional overlays show the slit width, the top and bottom edge paths to the probe, and the first-minimum guide when the current ratio lambda / a gives a finite first minimum.

Graph summary

The probe-field graph shows the oscillation at the slit center and the current probe point, with dashed envelope lines marking the local diffraction amplitude.

The screen-pattern graph shows relative intensity against screen height, so it remains a spatial map while the time rail inspects the local probe field.