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Diffraction

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Wrap-up

What you learned

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Open concept testCheck whether the core ideas are ready without leaving this concept.
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Double-Slit InterferencePut interference fringes inside the envelope

Key takeaway

  1. Diffraction spread is controlled mainly by the wavelength-to-slit ratio lambda / a.
  2. The first minimum gives a concrete marker for the width of the central bright region.
  3. Moving the probe samples a different screen point; it does not by itself change the pattern.
  4. Single-slit diffraction becomes the envelope that shapes later double-slit and imaging patterns.

Common misconception

A narrower opening does not make the outgoing beam narrower; when a becomes smaller relative to lambda, the diffraction peak spreads wider.

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

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Read next

Keep the sequence moving with the most relevant follow-up concept.

  1. Read nextPut interference fringes inside the envelopeDouble-Slit Interference
    1. Read nextUse aperture spread as an imaging limitOptical Resolution / Imaging Limits
    2. Read nextReview phase and path differenceWave Interference

Read the first minimum as the practical width marker, then use edge-path difference and intensity to explain why the graph gets bright or dim.

  1. Single-slit spread

    The first dark band appears when the edge-to-edge path difference reaches one wavelength.

  2. Edge-to-edge path difference

    The top and bottom edges of the slit send waves to the same screen point with slightly different path lengths.

  3. Single-slit intensity

    Gives the broad central peak and the weaker side peaks of a single-slit pattern.

Worked examples

Live diffraction checks

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Frozen walkthrough

Step through the frozen example

Frozen walkthrough
Use the live slit width, wavelength, and probe position from the stage. The calculation should explain the pattern you can already see, not replace it.

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Example 1 of 2
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. Use the first-minimum condition

Use the single-slit condition .

2. Substitute the live ratio lambda / a

.

3. Connect the result to the pattern width

.

First-minimum prediction

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 bright region to spread out without moving the screen. Which control change will reliably do it?

Make a prediction before you reveal the next step.

Choose between changing wavelength, slit width, and probe position.

Check your reasoning against the live bench.

Increase the wavelength or decrease the slit width so the ratio lambda / a gets larger.
Moving the probe only samples a different part of the existing pattern. The pattern itself spreads more only when the wavelength becomes larger relative to the opening or the opening becomes smaller relative to the wavelength.

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