Understanding Diffraction and Its Applications

Oct 16

3 min read

When light, sound, or any type of wave encounters an obstacle or a gap, it doesn’t always just stop or bounce back. Instead, the wave can bend around the edges or spread out after passing through an opening. This phenomenon is called diffraction.

Light being diffracted in glasses lenses

What is Diffraction?

Diffraction happens when waves encounter:

An edge (e.g. water waves bending around a harbour wall)
A slit or gap (e.g. light passing through a narrow slit and spreading out)
An obstacle (e.g. sound bending around buildings)

The amount of diffraction depends on the relationship between the wavelength (λ) of the wave and the size of the gap or obstacle (a):

The amount of diffraction is highest when the size of the gap is similar to the wavelength of the wave. If the gap larger than wavelength, diffraction is less, and the waves pass through with little spreading. If the gap is smaller than wavelength, the diffracted waves spread out more.

Relationship between size of gap, wavelength and extent of diffraction

Everyday Examples of Diffraction

Sound waves: You can hear someone talking even if they are around a corner, because long-wavelength sound waves diffract around obstacles.

Light waves: Normally light doesn’t diffract much (wavelength is very small), but under controlled conditions (like shining through a single slit or a diffraction grating), the spreading can be seen as bright and dark patterns.

Water waves: Waves spread out after passing through a harbour entrance — a very visible example of diffraction.

Scientific and Technological Applications

Diffraction is not just a textbook idea — it’s used in many areas of science and technology:

Diffraction Gratings in Spectroscopy

A diffraction grating is a surface with many closely spaced slits.
Light diffracts through the slits and produces a pattern of bright lines at specific angles.
Each wavelength is diffracted by a different amount, so white light is spread into its component colours.
Used to analyse chemical substances by their emission or absorption spectra.

X-Ray Diffraction (XRD)

X-rays have very short wavelengths, comparable to the spacing of atoms in crystals.
When X-rays strike a crystal, they are diffracted in a pattern that can be used to determine atomic structure.
Applications: discovering DNA’s double helix, analysing crystal lattices in materials science.

CDs, DVDs and Blu-Ray Discs

Data is stored as tiny pits on the disc surface.
Laser light diffracts off these pits and the reflected diffraction pattern is read to interpret the data.

Medical Imaging & Structural Biology

Diffraction techniques are used to study proteins and viruses at the molecular level.

Why is Diffraction Important in Applied Science?

In Unit 1, students need to understand wave behaviour and how scientific principles apply to real technologies. Diffraction is central to how we study the structure of matter, analyse chemicals, and design optical devices.

📝 Sample Exam Question (6 marks)

Question:

A learner shines light from a laser onto a diffraction grating, producing a pattern of light produced by the grating. Figure 1 shows the equipment used

Figure 1 - Diagram of diffraction experiment

Figure 2 shows the pattern produced on the screen

Figure 2 - Image of diffracted light on a screen

Explain how the light passing through the diffraction grating produces a pattern of light and dark regions on the screen.

Total: 6/6

Model Answer

First describe what diffraction is

A diffraction grating has many slits and as light passes through the slits the light is spread out (diffracted) by each slit.

Then explain what happens to the diffracted waves

Each ray of diffracted light overlaps with another on its way to the screen. Because the waves are coherent, they add together (a process called superposition). When waves are in phase when they add together, this causes constructive interference. When waves are out of phase when they add together, this causes destructive interference.

Explain why the pattern forms

Bright regions are produced on the screen where constructive interference occurs (path difference is one whole number of wavelengths) and a dark band appears where destructive interference occurs (path difference is a half number of wavelengths).

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