Interactive Syllabus Extract for

Section 3.3.2 - Refraction, diffraction and interference


The concept of path difference and coherence.
  AT i Investigation of two-source interference with sound, light and microwave radiation.

The laser as a source of coherent monochromatic light used to demonstrate interference and diffraction

Comparison with non-laser light;


Students will not be required to describe how a laser works. Students are expected to show awareness of safety issues associated with using lasers.

Young's double-slit experiment: the use of two coherent sources or the use of a single source with double slits to produce an interference pattern.

The appearance of the interference fringes produced by a double slit system

Requirements of two source and single source double-slit systems for the production of fringes.


w is the fringe spacing and

s is the slit separation

Required practical 2: Investigation of interference effects to include the Young’s slit experiment and interference by a diffraction grating.

Production of interference pattern using white light.


Students will be expected to describe and explain interference produced with sound and electromagnetic waves.

    Appreciation of how knowledge and understanding of nature of electromagnetic radiation has changed over time Diffraction Appearance of the diffraction pattern from a single slit using monochromatic and white light.

Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width.

The graph of intensity against angular separation is not required.

  The plane transmission diffraction grating at normal incidence

Use of the spectrometer will not be tested.

Optical details of the spectrometer will not be required.

  Derivation of the diffraction grating equation

Applications; e.g. spectral analysis of light from stars.

Where n is the order number. Refraction at a plane surface

Students should recall that the refractive index of air is approximately 1.


Candidates are not expected to recall methods for determining refractive indices.

Refractive index of a substance s:

Snell's Law of refraction at a boundary for a boundary between two different substances of refractive indices n1 and n2  
Total internal reflection including calculations of the critical angle at a boundary between a substance of refractive index n1 and a substance of lesser refractive
index n2 or air;

Simple treatment of fibre optics including function of the cladding with lower refractive index around central core - limited to step index only;

Material and modal dispersion.

Application to



Students are expected to understand the principles and consequences of pulse broadening and absorption.