DP IB Physics: HL

Revision Notes

Syllabus Edition

First teaching 2014

Last exams 2024


9.2.1 The Nature of Single-Slit Diffraction

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The Nature of Single-Slit Diffraction

  • When plane waves are incident normally on a single slit, a diffraction pattern is produced
    • This is represented as a series of light and dark fringes which show the areas of maximum and minimum intensity
  • If a laser emitting blue light is directed at a single slit, where the slit width is larger than the wavelength of the light, it will spread out as follows:


The intensity pattern of blue laser light diffracted through a single slit

  • The features of the single-slit diffraction pattern using monochromatic light are:
    • A central maximum with a high intensity
    • Equally spaced subsidiary maxima, successively smaller in intensity and half the width of the central maximum

Single Slit Diffraction of White Light

  • When white light is incident on a slit, separate diffraction patterns can be observed for each wavelength making up the white light


Single slit diffraction of a white light source

  • If the laser were to be replaced by a non-laser source emitting white light:
    • The central maximum would be white
    • All maxima would be composed of a spectrum
    • The shortest wavelength (violet / blue) would appear nearest to the central maximum
    • The longest wavelength (red) would appear furthest from the central maximum
    • The fringe spacing would be smaller and the maxima would be wider

White Light Diffraction

Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width. Red light is diffracted the most, blue light is diffracted the least.

Exam Tip

Be mindful that all waves undergo diffraction, so questions about diffraction may involve sound, ultrasound, electromagnetic waves, or even waves on water

Slit Width

  • The angle of diffraction is directly proportional to the wavelength of the light
    • This means that the width of the bright maxima, or fringe, is also proportional to the wavelength
  • Red light – which has the longest wavelength of visible light – will produce a diffraction pattern with wide fringes
  • Blue light – which has a much shorter wavelength – will produce a diffraction pattern with narrow fringes


Fringe width depends on the wavelength of the light

  • Therefore, if the blue laser were to be replaced with a red laser:
    • The wavelength of red light is longer so the light would diffract more
    • The intensity fringes would therefore be wider

Diffraction graph

The intensity pattern of red laser light shows longer wavelengths diffract more than shorter blue wavelengths

  • If the slit was made narrower:
    • The intensity would decrease
    • The fringe spacing would be wider

Exam Tip

When drawing diffracted waves, take care to keep the wavelength (the distance between each wavefront) constant. It is only the amplitude of the wave that changes when diffracted.

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Lindsay Gilmour

Author: Lindsay Gilmour

Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.