The VLBA (Very Long Baseline Array) uses a number of individual radio telescopes to make one unit having an equivalent diameter of about 8000 km. When this radio telescope is focusing radio waves of wavelength 2.0 cm, what would have to be the diameter of the mirror of a visible-light telescope focusing light of wavelength 550 nm so that the visible-light telescope has the same resolution as the radio telescope?
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34. Wave Optics
Diffraction
Problem 74
Textbook Question
Spy planes fly at extremely high altitudes (25 km) to avoid interception. If their cameras are to discern features as small as 5 cm, what is the minimum aperture of the camera lens to afford this resolution? (Use λ = 580nm.)

1
Understand the problem: The question involves resolving small features using a camera lens at a high altitude. This is a diffraction-limited resolution problem, which can be solved using the Rayleigh criterion. The Rayleigh criterion states that the minimum resolvable angle θ is given by θ = 1.22 * (λ / D), where λ is the wavelength of light and D is the aperture diameter.
Relate the angular resolution to the physical resolution: The angular resolution θ is related to the physical resolution (the smallest discernible feature size) and the distance to the object. Use the formula θ = s / L, where s is the smallest feature size (5 cm) and L is the distance to the object (25 km).
Combine the two equations: Substitute θ = s / L into the Rayleigh criterion formula θ = 1.22 * (λ / D). This gives s / L = 1.22 * (λ / D). Rearrange this equation to solve for D, the aperture diameter: D = 1.22 * (λ * L / s).
Substitute the known values: λ = 580 nm = 580 * 10^(-9) m, L = 25 km = 25 * 10^3 m, and s = 5 cm = 0.05 m. Plug these values into the formula D = 1.22 * (λ * L / s).
Simplify the expression: Perform the necessary unit conversions and simplify the expression to find the minimum aperture diameter D. This will give you the required aperture size for the camera lens to resolve features as small as 5 cm from a height of 25 km.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Resolution in Imaging
Resolution refers to the ability of an imaging system to distinguish between two closely spaced objects. In this context, it is defined by the smallest feature size that can be resolved by the camera, which is given as 5 cm. The resolution is influenced by factors such as the wavelength of light used and the aperture of the lens.
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Diffraction Limit
The diffraction limit is a fundamental principle in optics that describes the limit to the resolution of an imaging system due to the wave nature of light. It is determined by the wavelength of light and the diameter of the aperture. A smaller aperture increases the depth of field but can reduce the amount of light entering the camera, affecting image brightness and clarity.
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Aperture and F-number
The aperture of a camera lens is the opening that allows light to enter, and it is typically expressed in terms of the F-number (f-stop). The F-number is the ratio of the lens's focal length to the diameter of the aperture. A lower F-number indicates a larger aperture, allowing more light to enter and improving the potential resolution of the image, which is crucial for capturing fine details from high altitudes.
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