CAIE A-Level Physics 8.3.3 Conditions for Observable Interference

Practise identifying conditions for stable two-source interference, using path or phase difference to locate constructive and destructive interference.

Exam points

state the conditions required for stable two-source interference fringes, including coherent sources and a fixed phase relationship
use path difference or phase difference to identify positions of constructive and destructive interference
interpret an interference pattern to determine fringe spacing, order or phase relationship

[Maximum number: 3]

(a)

Interference fringes may be observed using a light-emitting laser to illuminate a double slit. The double slit acts as two sources of light.

Explain

[ 1 ]

(i)

the reason why a double slit is used rather than two separate sources of light.

[ 1 ]

(b)

Two dippers $\mathrm{D}_{1}$ and $\mathrm{D}_{2}$ are used to produce identical waves on the surface of water, as illustrated in Fig. 6.3.

Fig. 6.3 (not to scale)

Point P is 7.2 cm from $D_{1}$ and 11.2 cm from $D_{2}$ .
The wavelength of the waves is 1.6 cm . The phase difference between the waves produced at $\mathrm{D}_{1}$ and $\mathrm{D}_{2}$ is zero.

[ 2 ]

(i)

State and explain the effect on the answer to (c)(i) if the apparatus is changed so that, separately,
1. the phase difference between the waves at $\mathrm{D}_{1}$ and at $\mathrm{D}_{2}$ is $180^{\circ}$ ,
2. the intensity of the wave from $D_{1}$ is less than the intensity of that from $D_{2}$ .

[ 2 ]

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8.3.3—The conditions required if two-source interference fringes are to be question 1

Question (a)

Question (i)

Question (b)

Question (i)