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8.3. Interference
A subsection of Physics, 9702, through 8. Superposition
Listing 10 of 90 questions
State three advantages of an optic fibre compared to a metal wire for the transmission of a signal. 1. 2. 3. An optic fibre of length 57 km is connected between a transmitter and a receiver, as shown in . signal power 15 × 10–3 W transmitter receiver optic fibre signal power P noise power 9.0 × 10–7 W 57 km The attenuation per unit length of the optic fibre is 0.50 dB km–1. The transmitter provides an input signal of power 15 × 10–3 W to the fibre. The noise power at the receiver is 9.0 × 10–7 W. Show that the signal power P entering the receiver from the optic fibre is 2.1 × 10–5 W. A minimum signal-to-noise ratio of 24 dB is needed at the receiver in order for it to be able to distinguish the signal from the noise. Determine whether the receiver is able to distinguish the signal from the noise.
9702_m17_qp_42
THEORY
2017
Paper 4, Variant 2
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By reference to two waves, state: the principle of superposition what is meant by coherence. Two coherent waves P and Q meet at a point in phase and superpose. Wave P has an amplitude of 1.5 cm and intensity I. The resultant intensity at the point where the waves meet is 3I. Calculate the amplitude of wave Q. amplitude = cm The apparatus shown in is used to produce an interference pattern on a screen. a D double-slit laser light wavelength 680 nm screen (not to scale) Light of wavelength 680 nm is incident on a double-slit. The slit separation is a. The separation between adjacent fringes is x. Fringes are viewed on a screen at distance D from the double-slit. Distance D is varied from 2.0 m to 3.5 m. The variation with D of x is shown in . 2.0 2.0 4.0 x / mm D / m 8.0 6.0 10.0 2.5 3.0 3.5 Use to determine the slit separation a. a = m The laser is now replaced by another laser that emits light of a shorter wavelength. On , sketch a possible line to show the variation with D of x for the fringes that are now produced.
9702_m19_qp_22
THEORY
2019
Paper 2, Variant 2
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Coherent visible light of a single frequency is incident normally on a double slit. This produces a pattern of bright and dark interference fringes on a screen, as illustrated in . 10.2 mm 3.1 m 1.2 mm light double slit fringe pattern on screen screen bright fringe X bright fringe Y dark fringe (not to scale) There are seven bright fringes. Explain how the pattern of bright and dark interference fringes is formed. The distance between the centres of bright fringe X and bright fringe Y in the pattern is 10.2 mm. The slit spacing is 1.2 mm. The distance from the slits to the screen is 3.1 m. Calculate the wavelength of the light incident on the slits. wavelength = m The light is replaced by different visible light with a shorter wavelength. State how the new fringe separation will compare to the original fringe separation. A stationary wave is formed on a stretched string AB, as shown in . B A R P Q string mean position of string P, Q and R are points on the string. On Fig. 6. 2, draw a cross (×) to show the position of a node. State the phase difference between P and Q. phase difference = ° State the phase difference between P and R. phase difference = °
9702_m24_qp_22
THEORY
2024
Paper 2, Variant 2
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12 Optic fibre transmission has, in some instances, replaced transmission using co-axial cables and wire pairs. Optic fibres have negligible cross-talk and are less noisy than co-axial cables. Explain what is meant by cross-talk, noise. An optic fibre has a signal attenuation of 0.20 dB km–1. The input signal to the optic fibre has a power of 26 mW. The receiver at the output of the fibre has a noise power of 6.5 µW. Calculate the maximum uninterrupted length of optic fibre given that the signal-to-noise ratio at the receiver must not be less than 30 dB. length = km
9702_s08_qp_4
THEORY
2008
Paper 4, Variant 0
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12 A telephone link between two towns is to be provided using an optic fibre. The length of the optic fibre between the two towns is 75 km. State two changes that occur in a signal as it is transmitted along an optic fibre. 1. 2. The optic fibre has an attenuation per unit length of 1.6 dB km–1. The minimum permissible signal-to-noise power ratio in the fibre is 25 dB. The average noise power in the optic fibre is 6.1 × 10–19 W. Suggest one reason why power ratios are expressed in dB. The signal input power to the optic fibre is designed to be 6.5 mW. Determine whether repeater amplifiers are necessary in the optic fibre between the two towns.
9702_s10_qp_42
THEORY
2010
Paper 4, Variant 2
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Questions Discovered
90