 # Interference for Edexcel A-level Physics 1. Path difference
2. Fringe spacing
3. 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. 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

If two keys on a piano are played simultaneously there is no interference experienced. Suggest why this is the case.

Human hearing can only detect interference when the frequency of both sounds are the same, or very similar, so two piano keys are too different to create the beats of interference. # 4

A version of the double slit experiment has been set up using light of a wavelength of 2.5 × 10⁻⁷ m as shown in the diagram. Calculate the distance between the observation screen and the slits.

D = ws ÷ λ = (3 × 10⁻³ × 0.5 ×10⁻³) ÷ (2.5 × 10⁻⁷) = 6 m. # 5

The interference pattern of light waves is observed as fringes of bright and dark bands of light. How is the interference pattern of sound waves different to that of light?

Sound waves produce an interference pattern that is similar to water waves and light waves, except the interference is shown by fringes of loud and quite sound, rather than a pattern of light and dark. This is experienced as beats of sound. End of page