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Problem 3: The following problem helps you to understand the nature of wave packets and light pulses. In this problem, the wave packet is trivial: it contains only two frequencies. In general, however, it would

contain infinitely many frequencies. That case is then dealt with the help of the Fourier transform, which will be looked at in the next problem Consider a non-monochromatic wave (in this case a superposition of just two monochromatic waves) travelling in positive z-direction u(z, t) = a cos(2 it-k, z) + a cos(2n,t-k, z) For simplicity, assume that you are monitoring the oscillation of the wave at z-0. Assume a-3.5 (units of W1/2/cm), vı = 1 PHz , v,-1.05 PHz, and assume that the time is given in units offs. Make two plots First, plot the two monochromatic oscillations as function of time in the interval between 0 and the out of-phase time t Next, plot the total oscillation u(0,t) together with an envelope over the time interval 0 to 3t. Describe what you see in words. What would you expect if you were to plot the oscillation on a much larger time interval?

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0.2 0.4 0.6 0.8 Time (ps)

This is the graph for the first part of the wave, ie. wave with only 1 PHz frequency.

0.2 0.4 0.6 0.8 Time (ps)

This is the second part, i.e. wave with 1.05 PHz frequency.

We can show here that these are out of phase in the following diagram:

2) apmi duly

Here are the two independent waves drawn on same graph, showing how these are out of phase.

Now, we can show the total wave u(0,t) by following diagram:

20 10 10 Time (ps) -10 10 20

We can clearly see that a wavepacket appears. The envelope is formed by meeting the top most points together, and then meeting the lower most points.

This pattern will keep on repeating at much larger time intervals.

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