 # Interference for AQA A-level Physics 1. Path difference
2. Young's double slit experiment
3. Fringe spacing
4. Interference patterns

When talking about phase difference (φ) we can be referring to two separate waves, or two particles in the same wave, and it tells us how far or behind one wave/ particle is from another. Path difference also refers to different waves, but it refers to the difference in the distance that the two waves have travelled. If two waves overlap then this is known as interference, interference can either be constructive or destructive, depending on the phase difference of the waves. The path difference can be used to determine the phase difference of two waves using the following calculation. The double slit experiment, as shown in the diagram, can be used to create an interference pattern on a screen or it can be detected using a suitable wave detector. Light from a single source travels through each slit, and they then act as two coherent sources of light. Light diffracts through each slit and the interference of the two sources can be recorded in order to observe areas of constructive and destructive interference. The double slit experiment is often done with a light source so that the ‘fringes’ or strips of light created by the two sources can be observed on a screen; the intensity of the light in each fringe can be used to create a graph. During the double slit experiment, light waves diffract and interfere with one another to form an interference pattern on an observation screen. This interference pattern is bands of alternating dark and light strips known as fringes. Different light sources create different fringe spacings and the fringe spacing of a particular experiment can be used to calculate the wavelength of the light, and it is shown in the diagram. When light travels through a double slit experiment the interference experienced by the light waves creates an interference pattern. There are areas of constructive and destructive interference that are plotted onto a graph to create the pattern shown in the diagram. # 1

Which points in the provided diagram have a phase difference of π rad from point C?

B, E # 2

What is meant by the term ‘interference pattern’?

An interference pattern is usually seen in the double slit experiment and is defined as a series of alternating dark and bright bands of light that are produced through light interference. # 3

Microwave radiation with a wavelength of 30 mm is used to exhibit the double slit experiment, and the waves are detected using a microwave detector. Which of the following path distances would produce constructive interference? A. 15 mm. B. 30 mm. C. 45 mm.

b. 30 mm # 4

Light waves with a wavelength of 0.04 m are used in the double slit experiment and the intensity of the resulting fringes of light are recorded by a lux meter. The lux meter is moved between points G and H and it measures a number of minima and maxima in light intensity. The distance from slit A to point X is 3.22 m, and the distance from slit B to point X is 3.06 m. What type of interference is detected at point X?

Find the path difference between AX and BX: Path difference = AX – BX = 3.22 – 3.06 = 0.16 m. Number of wavelengths in that path difference = 0.16 ÷ 0.04 = 4. There is an even number of wavelengths in the path difference, therefore the interference is constructive. # 5

If the brightness of the light source is reduced what effect does that have on the fringes shown on the observation screen?

The spacing of the fringes in unaffected, but the fringes shown will be duller, or less intense. End of page