Diffraction, interference & diffraction gratings
When waves meet obstacles and each other, something beautiful happens. In this AS-level lesson we build the physics of superposition step by step: diffraction — how a wave spreads out through a gap, and why the spreading is greatest when the gap is about the size of the wavelength; interference — how two waves add to give bright and dark regions, and the meaning of coherence; the path-difference conditions for constructive (nλ) and destructive ((n+½)λ) interference; Young's double-slit experiment and the equation λ = ax/D; and the diffraction grating equation d sinθ = nλ, including how to find the slit spacing d from lines per metre. We finish by comparing the double-slit and grating patterns, with fully worked examples throughout.
By the end you should be able to (NSSCAS Physics (AS) 2.6.2):
- Explain the meaning of the term diffraction
- Describe experiments that demonstrate diffraction, including the qualitative effect of the gap width relative to the wavelength of the wave (e.g. water waves in a ripple tank)
- Define the terms interference and coherence
- Describe experiments that demonstrate two-source interference using water ripples, light (a monochromatic source e.g. laser) and microwaves
- Discuss the conditions required if two-source interference fringes are to be observed
- Relate path difference to constructive interference (path difference = nλ) and destructive interference (path difference = (n + ½)λ)
- Recall and solve problems using the equation λ = ax/D for double-slit interference using light
- Recall and solve problems using the diffraction grating formula d sinθ = nλ, and find d from the number of lines per metre
- Describe the use of a diffraction grating to determine the wavelength of light, and compare the double-slit and grating patterns
Miss Elizabeth and Mike talk through the whole topic — with the figure and working drawn live.