What minimum bandwidth is needed to transmit a pulse that consists of 100 cycles of a 1.0 MHz oscillation?
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35. Special Relativity
Inertial Reference Frames
Problem 40d
Textbook Question
A pulse of light is created by the superposition of many waves that span the frequency range f₀ − (1/2) Δf ≤ f ≤ f₀ + (1/2) Δf, where f₀ = c/λ is called the center frequency of the pulse. Laser technology can generate a pulse of light that has a wavelength of 600 nm and lasts a mere 6.0 fs (1 fs = 1 femtosecond =10−15 s). What is the spatial length of the laser pulse as it travels through space?

1
Step 1: Understand the problem. The spatial length of the laser pulse is the distance the pulse travels in space during its duration. This can be calculated using the relationship between speed, time, and distance: distance = speed × time. Here, the speed of the pulse is the speed of light in a vacuum, c = 3.0 × 10⁸ m/s, and the duration of the pulse is given as 6.0 fs (femtoseconds).
Step 2: Convert the duration of the pulse from femtoseconds to seconds. Since 1 femtosecond (fs) = 10⁻¹⁵ seconds, multiply the given duration (6.0 fs) by 10⁻¹⁵ to express it in seconds.
Step 3: Use the formula for distance: distance = speed × time. Substitute the speed of light (c = 3.0 × 10⁸ m/s) and the duration of the pulse (in seconds, calculated in Step 2) into the formula.
Step 4: Perform the multiplication to find the spatial length of the laser pulse. This will give the distance the pulse travels in meters during its duration.
Step 5: If needed, convert the result into a more convenient unit (e.g., nanometers or micrometers) for better interpretation, depending on the context of the problem.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Pulse of Light
A pulse of light is a short burst of electromagnetic radiation characterized by its duration and frequency range. It can be described as a superposition of multiple waves, each with different frequencies, which together create a localized wave packet. The center frequency, f₀, represents the average frequency of the pulse, while the bandwidth, Δf, indicates the range of frequencies present.
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Wavelength and Frequency Relationship
The relationship between wavelength (λ) and frequency (f) of light is given by the equation f = c/λ, where c is the speed of light in a vacuum. This relationship implies that as the wavelength decreases, the frequency increases, and vice versa. Understanding this relationship is crucial for determining the properties of light pulses, including their spatial characteristics.
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Spatial Length of a Pulse
The spatial length of a light pulse can be calculated using the product of its speed and duration. Since light travels at a constant speed (c), the length of the pulse as it propagates through space can be found by multiplying the speed of light by the pulse duration. For a pulse lasting 6.0 femtoseconds, this calculation provides insight into how far the pulse travels during its brief existence.
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