Understandings: The nature of single-slit diffraction
Applications and skills: Describing the effect of slit width on the diffraction pattern, Determining the position of first interference minimum, Qualitatively describing single-slit diffraction patterns produced from white light and from a range of monochromatic light frequencies
Guidance: Only rectangular slits need to be considered, Diffraction around an object (rather than through a slit) does not need to be considered in this sub-topic (see Physics sub-topic 4.4), Students will be expected to be aware of the approximate ratios of successive intensity maxima for single-slit interference patterns, Calculations will be limited to a determination of the position of the first minimum for single-slit interference patterns using the approximation equation
When a wavefront reaches a narrow aperture then the wave diffracts. The pattern it produces on a screen is called a single-slit diffraction pattern.To understand why this pattern is produced we must understand the nature of wave propagation. Every point on a wave can be thought of as a source of a wave of the same wavelength. These wavelets combine (by superposition) to form the next wavefront. Thus a linear wave will propagate as a linear wave and a circular wave will propagate as a circular wave. This idea is known as Huygens's construction. An narrow aperture will therefore act as a near point source, and the light will diffract in a semi-circular pattern. However, no aperture is perfectly narrow - and sources of wavelets along that wavefront will interfere with each offer to produce the single-slit diffraction pattern. The formula to calculate the position of the first minima (the gaps either side of the central maximum) is b.sinθ=λ where b is the slit width, θ is the angle to the first minima and λ is the wavelength of the light source. This phenomenon is most easily seen using a monochromatic, coherent light source such as a laser.
The great Walter Fendt has an interactive simulation of single-slit diffraction.
Oxford Physics: pages 364 - 367, there is a good worked example on page 366. See also past IB questions (4 and 5) on page 388.
Hamper HL (2014): pages 189 - 192
Pages 99 - 102