Module 8: Sinusoidal Waves Lecture 8: Sinusoidal Waves

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1 Module 8: Sinusoidal Waves Lecture 8: Sinusoidal Waves We shift our attention to oscillations that propagate in space as time evolves. This is referred to as a wave. The sinusoidal wave a(,t) = A cos(ωt k + ψ), (8.1) is the simplest eample of a wave, we shall consider other possibilities later in the course. It is often convenient to represent the wave in the comple notation introduced earlier. We have ã(,t) = Ãei(ωt k). (8.2) 8.1 What is a(, t)? The wave phenomena is found in many different situations, and a(,t) represents a different physical quantity in each situation. For eample, it is well known that disturbances in air propagate from one point to another as waves and are perceived by us as sound. Any source of sound (eg. a loud speaker) produces compressions and rarefactions in the air, and the patterns of compressions and rarefactions propagate from one point to another. Using ρ(,t) to denote the air density, we can epress this as ρ(,t) = ρ + ρ(,t) where ρ is the density in the absence of the disturbance and ρ(,t) is the change due to the disturbance. We can use equation (8.1) to represent a sinusoidal sound wave if we identify a(,t) with ρ(,t). The transverse vibrations of a stretched string is another eamples. In this situation a(,t) corresponds to y(,t) which is the displacement of the string shown in Figure Angular frequency and wave number The sinusoidal wave in equation (8.2) has a comple amplitude à = Aeiψ. Here A, the magnitude of à determines the magnitude of the wave. We refer 49

2 50 CHAPTER 8. SINUSOIDAL WAVES y Figure 8.1: Transverse vibrations of a stretched string. to φ(,t) = ωt k + ψ as the phase of the wave, and the wave can be also epressed as ã(,t) = Ae iφ(,t), (8.3) If we study the behaviour of the wave at a fied position 1, we have ã(t) = [Ãe ik 1 ]e iωt = à e iωt. (8.4) We see that this is the familiar oscillation (SHO) discussed in detail in Chapter 1. The oscillation has amplitude à = [Ãe ik 1 ] which includes an etra constant phase factor. The value of a(t) has sinusoidal variations. Starting at t = 0, the behaviour repeats after a time period T when ωt = 2π. We identify ω as the angular frequency of the wave related to the frequency ν as ω = 2π T = 2πν. (8.5) We net study the wave as a function of position at a fied instant of time t 1. We have ã() = Ãeiωt 1 e ik = à e ik, (8.6) where we have absorbed the etra phase e iωt in the comple amplitude Ã. This tells us that the spatial variation is also sinusoidal as shown in Figure 8.2. The wavelength λ is the distance after which a() repeats itself. Starting from = 0, we see that a() repeats when k = 2π which tells us that kλ = 2π or k = 2π λ, (8.7) where we refer to k as the wave number. We note that the wave number and the angular frequency tell us the rate of change of the phase φ(,t) with position and time respectively k = φ andω = φ t. (8.8)

3 8.3. PHASE VELOCITY 51 a() λ Figure 8.2: Variation of wave with space for a fied time φ=0 a(,0) a(, t) Figure 8.3: The movement of constant phase φ = 0, as function of time. 8.3 Phase velocity We now consider the evolution of the wave in both position and time together. We consider the wave ã(,t) = Ae i(ωt k), (8.9) which has phase φ(,t) = ωt k. Let us follow the motion of the position where the phase has value φ(,t) = 0 as time increases. We see that initially φ = 0 at = 0,t = 0 and after a time t this moves to a position ( ) ω = t, (8.10) k shown in Figure 8.3. The point with phase φ = 0 moves at speed ( ) ω v p =. (8.11) k It is not difficult to convince oneself that this is true for any constant value of the phase, and the whole sinusoidal pattern propagates along the + direction (Figure 8.4) at the speed v p which is called the phase velocity of the wave.

4 52 CHAPTER 8. SINUSOIDAL WAVES a t= t=0 t= Figure 8.4: Propagation of wave along the -direction. 8.4 Waves in three dimensions We have till now considered waves which depend on only one position coordinate and time t. This is quite adequate when considering waves on a string as the position along a string can be described by a single coordinate. It is necessary to bring three spatial coordinates (,y,z) into the picture when considering a wave propagating in three dimensional space. A sound wave propagating in air is an eample. We use the vector r = î + yĵ + zˆk to denote a point in three dimensional space. The solution which we have been discussing ã( r,t) = Ae i(ωt k), (8.12) can be interpreted in the contet of a three dimensional space. Note that ã( r,t) varies only along the direction and not along y and z. Considering the phase φ( r,t) = ωt k we see that at any particular instant of time t, there are surfaces on which the phase is constant. The constant phase surfaces of a wave are called wave fronts. In this case the wave fronts are parallel to the y z plane as shown in Figure 8.5. The wave fronts move along the + direction with speed v p as time evolves. You can check this by following the motion of the φ = 0 surface shown in Figure Waves in an arbitrary direction Let us now discuss how to describe a sinusoidal plane wave in an arbitrary direction denoted by the unit vector n. A wave propagating along the î direction can be written as ã( r,t) = Ãei(ωt k r), (8.13) where k = kî is called the wave vector. Note that k is different from ˆk which is the unit vector along the z direction. It is now obvious that a wave along an arbitrary direction ˆn can also be represented by eq. (8.13) if we change the

5 8.5. WAVES IN AN ARBITRARY DIRECTION 53 t=0 y φ=0 φ=1 φ=2 z y t=1 φ=0 φ=1 φ=2 z Figure 8.5: Wavefronts for a wave propagating along the direction. n r Figure 8.6: Wavefronts for a wave propagating along the direction n. wave vector to k = kˆn. The wave vector k carries information about both the wavelength λ and the direction of propagation ˆn. For such a wave, at a fied instant of time, the phase φ( r,t) = ωt k r changes only along ˆn. The wave fronts are surfaces perpendicular to ˆn as shown in Figure 8.6. Problem: Show the above fact, that is the surface swapped by a constant phase at a fied instant is a two dimensional plane and the wave vector k is normal to that plane. The phase difference between two point (shown in Figure 8.6) separated by r is φ = k r. Problems 1. What are the wave number and angular frequency of the wave a(,t) = A cos 2 (2 3t) where and t are in m and s respectively? (4 m 1, 6 s 1 ) 2. What is the wavelength correspnding to the wave vector k = 3î+4ĵ m 1? (0.4π m)

6 54 CHAPTER 8. SINUSOIDAL WAVES 3. A wave with ω = 10 s 1 and k = 7î + 6ĵ 3ˆk m 1 has phase φ = π/3 at the point (0, 0, 0) at t = 0. [a.] At what time will this value of phase reach the point (1, 1, 1) m? [b.] What is the phase at the point (1, 0, 0) m at t = 1 s? [c.] What is the phase velocity of the wave? ([a.] 1 s [b.] 4.05 rad [c.] 1.03 m s 1 4. For a wave with k = (4î + 5ĵ)m 1 and ω = 10 8 s 1, what are the values of the following? [a.] wavelength, [b.] frequency [c.] phase velocity, [d.] phase difference between the two points (,y,z) = (3, 4, 7) m and (4, 2, 8) m. 5. The phase of a plane wave is the same at the points (2, 7, 5), (3, 10, 6) and (4, 12, 5) and the phase is π/2 ahead at (3, 7, 5). Determine the wave vector for the wave.[all coordinates are in m.] 6. Two waves of the same frequency have wave vectors k 1 = 3î + 4ĵ m 1 and k 1 = 4î + 3ĵ m 1 respectively. The two waves have the same phase at the point (2, 7, 8) m, what is the phase difference between the waves at the point (3, 5, 8) m? (3 rad)